New notifications area at the top-right corner
Fix thumbnails for transparent images
Timeline: make selected range outline a little bigger
Fix problems generating mouse messages with pos=0,0
Fix issue loading negative splitter position
Add support to modify observers list when we are inside a notification
ButtonSet: fix a bug using setSelectedItem() when there are no selected items
ButtonSet: Add deselectItems() member function
ContextBar: Add buttons to drop the selection or cancel the whole operation

- Add ContextBarObserver (and MovingPixelsState implements this interface)
- PixelMovements::discardImage() receives a "commit" flag to do a rollback
  of the operation if the user needs it.