Introduce ContainmentLayoutManager QML plugin

Summary:
This QML plugin offers a reusable, C++ based framework to manage layout
of items (mainly plasmoids but other stuff too) inside Plasma Containments
This will replace the Javascript based plasmoid layout manager inside the
FolderView and Desktop containments (is planned also forPlasma mobile homescreen)
This makes folderview code simpler and layouting more reliable.
Relayouting behavior after screen resolution change improved too, making easier to
keep a similar looking layout

Test Plan:
Tested on FolderView, a minimal containment using it may be shipped on the store
if we don't feel to ship the folderview port immediately

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22034