Add DocumentApi::undoEnabled()

Now we can disable the undo information in DocumentApi() passing
a NULL undoers collection on its ctor.
Simple implementation of issue #17 - Export animation to JSON/sprite sheet files

This is a good start point. From here we can add more formats and options
(XML, custom formats, templates, etc.), rotated sprites, trim, etc.)
Moved Transparent Color button to ContextBar

Implemented SelectionSettings class

Transparent Color button for moving pixels moved from status bar to
context bar.  Current Transparent Color stored by new SelectionSettings
class, retrievable from ISettings.

Also implemented Observer classes for existing Settings interfaces for
future use.

StatusBarObserver interface and supporting code removed from StatusBar.