[FEATURE] Implemented the first "somewhat usable" Level of Detail functionality

Now you can Enable/Disable it via View->Fast Preview Mode (LOD) button
(or just press Shift+L).

Please note that when you switch on/off, Krita should synchronize all
the caches, so it hangs up of a couple of seconds to render all unfinished
strokes.

CC:kimageshop@kde.org