Improve thumbnail integration in qtquick
In order to share window thumbnail textures between kwin's scene graph and qtquick, we need a global share context. On Wayland, that's already the case. On X11, we need to add support for that.
The goal of this merge request is to improve the integration of thumbnail items and make WindowThumbnailItem suitable for implementing a qml replacement for the desktop grid and the present windows effects.
WindowThumbnailItem and DesktopThumbnailItem act as texture providers.
cc #30 closes #25 (closed)
Edited by Vlad Zahorodnii