[Core][Window] Window content cache for inactive windows

[gdt050579]

Add e method to cache the content of a window (that is inactive) the first time it is being drawn in a Canvas style object. / CharacterBuffer.
Once the content is cached, request to OnPaint method will not redraw the Window and its childent, but only the cache.
This will significally speed up the performance (especially if multiple window are being opened).
The cache must be invalidated on certain situations like:

• the window become active
• the window is moved / resized
• one control from the window has OnFrame callback activate and might receive notification to redraw itself