Patch for plasmoid subsystem(containments/desktop) in plasma-desktop

Summary:
Under T5144

Set of pathes to fix plasmoid background mechanics.
Fixed:
Bug: 375307
Sometimes Plasmoid doesn't free all his space in LayoutManager

Bug: 375308
Plasmoid applet doesn't have maximum size handlers

Bug: 375349
Plasmoid can't be resized to declared minimumWidth

= A detailed explanation of bugs, fixes and mechanics changes =

Before anything else I'd like to explain why I need this patch. At winter holidays I had a free time and decided to rewrite plasma-simplemonitor(maded by dhabyx) - port it to plasma5 and improve it. As one of new features I added "skins" - posibility to change UI looks. Let's describe one skin as "row" and other as "column". Skins had different default sizes and different minimum sizes. As behaviour it sugges that when user change one skin to another plasmoid size supposed to change to default size of new skin. In plasma5 there is no resize function in plasmoid anymore. Instead docs pointed to use Layout properties. And so I tried to set maximum sizes in Layout. It doesn't work. When researching this problem it was occurred that there is no such functionality in plasmoids despite that plasmoid already uses Layout minimum sizes. And so I made this patch to fix that. When debugging this patch I found other bugs and fixed them alongside.

**To read next you should understand QML and Qt/C++. Especially what is QPropertyAnimation, how it works and behave over object properties.**
I would mention bugs in order of it's difficulties and their involvement in patch. From easy to hard.
  1. Bug 375349 (Plasmoid can't be resized to declared minimumWidth).
This bug was because of mess in size relationship. Because of it plasmoid wouldn't get smaller than Layout.minimumWidth + 2 * handleWidth. Bug was in appletItem minimumWidth calculation and in AppletHandler.qml resize calculation(resizeHandle.onPositionChanged) .
  2. Bug 375307 (Sometimes Plasmoid doesn't free all his space in LayoutManager),
Bug happens because of appletItem size animations. Let's see how bug happening. User is done resizing appletItem, after that appletItem would be resized and positioned in layoutManager to fit cell politics, result sizes and position would be locked in layoutManager as area which used by this plasmoid. Now will start animation over applet height, width, x and y. Guess what would happend if somebody tried to move plasmoid or resize it in exact that moment? Yup. appletItem would try to release area in layoutManager with current(still animated) properties which != end properies. And it would lead to some area in layoutManager to not release properly.
I did many things(for example tried to implement and use "end" properties) to patch this but I think that it should be fully patched with implemtation of appletItem.floating property and functions positionItem and releasePosition which now handle all operations over layoutManager to lock\unlock area.
  3. Bug 375308 (Plasmoid applet doesn't have maximum size handlers)
When implementing this patch I encountered several questions:
  - What to do in case of minimum > maximum size?
I think that when this happened the other restrictive size should be changed to match new one. So I added check to implemet this behaviour to handlers of restrictive sizes.
  - What to do with qreal -> int convertation for maxinum size?
Default maxsize is Number.POSITIVE_INFINITY and that it would be autoconverted into int.NEGATIVE_MAX_VALUE which is not good. So I added simple check to see if qreal value would be too much to handle in int. And if it so to change it to 1000000. I don't have any better idea how to handle this.
  - What to do when appletItem size set by layoutManager would be > than maximumSize?
I created "innerAppletItem" in face of mouseListener. To handle all of animations I created additional properties like endHeight, endX, etc. "End" - because they symbolise what height, x, and other would be at end of animation. With these additional properties current end values always known and using them "innerAppletItem" can handle all kind of size and position changes even when it animations are running.
  - What to do when plasmoid is moved to new place and animations is on?
Because of this I just can not write in "innerAppletItem"(mouseListener) "anchors.centerIn: parent". So I implemented x and y change handlers in appletItem which would move position of "innerAppletItem" to place of old center(where it was according to appletItem.parent coordinates) and then play animation of move to a new place.

Test Plan: Tested with various plasmoids on single monitor. Worked well.

Reviewers: davidedmundson, #plasma

Reviewed By: davidedmundson, #plasma

Subscribers: mart, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4204