New qquick item in PlasmaCore to render a live updating window
thumbnail. The implementation uses XCB to redirect the specified
window using the composite extension. This means a running compositor
is not required. Through the damage extension the item tracks changes
to the window and triggers updates of the texture. Furthermore the
item tracks geometry changes of the window to recreate the window
pixmap.
    
If the pixmap of the window is valid, a texture is generated from it
using the glx texture from pixmap extension. For this a new optional
dependency for glx is added. On platform where glx is not available
(e.g. Windows, Linux with OpenGL ES) this will not get compiled and
the window's icon is used instead as a fallback.

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For more info see also the documentation of the item in the header file.

I plan to also add an egl variant (which is magnitudes easier) once I figure out how to compile Qt to use egl instead of glx for the xcb platform plugin.