SelectAccelerator: Focus the KeyField after any modification is done

E.g. if a modifier checkbox is pressed, we want the keyboard focus back
to the KeyField (which is the only interesting field)
Fix more delays when we start drawing (related to issue #239)

Add an ImageBufferPtr parameter to RenderEngine::renderSprite() to avoid
allocating memory for each render.
Convert CenterOnZoom into a ISettings property
Fix Profile flags for MSVC in CMakeLists.txt
Fix issue #548 - Avoid moving hidden layers or auto-selecting them
Add --ignore-empty command line option (close #549)

This option can be used to avoid exporting empty frames/cels.
Final fix for "Paste at original position" (issue #57 and fix #336)