rudimentary implementation of animated layers
added support for playback of all animated layers
exporting multiple layers properly to png
loading and saving multiple layers support
fix bug where frames didnt update on load
initial support for keeping layers in correct order
Merge branch 'master' into multi-layer

Conflicts:
	lib/animation.py

	modified:   gui/animationwindow.py
	modified:   lib/anicommand.py
	modified:   lib/anilayers.py
	modified:   lib/animation.py
	modified:   lib/document.py
	modified:   lib/framelist.py
mypaint layer names now automatically managed
fix error on remove last layer
making proper use of command stack, strict layer order, initial undo support for layers, various optimizations
redid custom classes, now with timeline class containing all information and using command stack better
added Left/Right shortcut for changing layers
fixed saving/loading to support new format
introduction of new timeline window based on that by z-uo. allows more fluid view of all working frames and layers. still VERY buggy and much needs implementing
added animation properties dialog
layer descriptions and some fixes
minor improvements and cleanup
improved dividing lines
playback controls and improved timeline scrolling
infinite scrolling