Allow to paint over sprites

You need to select any image layer in layer tree and select tool "Brush"
(without icon yet)
Use pressure
Auto-expand surface ("infinite surface")
Fix shape of brush
Allow to undo paint (action LayerPaint)
Optimize memory usage for history of paint action. 

Inserting paint actions into middle or history not supported!
(segmentation fault)
Saving of modified sprites
Derive two new classes from Layer_PasteCanvas - Group and Switch
Update references to Layer_PasteCanvas
New action "Group Layers into Switch"
New action "Make New Frame"
Layer_Switch::get_current_layer() method
Split "Make New Frame" action to "Make New Frame" and "Layer Copy"
Select layer when Switch created
Allow to paint at Switch
Improve ValueDescSet. Allow to animate param without animation mode.
Action "Make New Frame" works without animation mode
Enable animation bar for brush
Wrap imported images by Switch Group. +Fix groups
Add optional 'description' param to LayerEncapsulate action