Introduce a KWin internal on-screen-notification service

Summary:
Recently we noticed that there are multiple areas where KWin needs to
inform the user about how to operate. Examples are:
* Screenshot
* ColorPicker
* Pointer constraint enabled
* Pointer constraint about to be removed
* Kill Window

For Screenshot and ColorPicker we used an EffectFrame to render it. But
this is not an optimal solution as it's lacking many features we would
need. We cannot properly use it from within KWin core, we cannot
implement features like hide on mouse over, etc. etc.

This change introduces an OnScreenNotification which supports:
* showing an icon
* showing a message
* timeout

It is Qml styled, so that it can be easily adjusted. This is a big
improvement over the EffectFrame solution. The Qml file creates a Plasma
Dialog of type OSD. Thus KWin places it like the normal OSD windows and
also looks kind of similar. In the case of KWin the focus is more on the
message, than an icon, so the icon is placed left of the text.

While the OnScreenNotification is supposed to be used like a singleton,
it doesn't use the KWin singleton pattern. Instead a small wrapper
namespace OSD is introduced which provides a convenient API for KWin
internal areas to show/hide the notification. By not using the KWin
singleton pattern, the OnScreenNotification does not depend on any other
parts of KWin and can be easily unit-tested.

A few features are still missing and will be added in further commits:
* hide-out on mouse over
* optional skip close animation (needed for screenshot)
* X11 support (not that important as it's mostly for Wayland features)

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3723