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"
Remove commented code
Remove unused code
Move Panels from File to main menubar as a mainmenu item instead of submenu item.
Rename (menu) Panels to Panel
Move Properties menu item from Edit to Canvas, since it is properties of canvas.
Remove close window menuitem from File menu.
Remove unused code
Remove unused code
Remove close view action (unuse anymore)