Improve undo-redo

[ReMinoer]

I made some improvements on the undo redo system:

• Allow to override a new protected method Document.CreateUndoRedoManager to change the implementation of IUndoRedoManager.
• Add method IUndoRedoManager.PushAction to stack an action without executing it.
  • Interesting when the first action implementation (the "do") is different from the redo, or executed before the action is stacked (for example, a painting brush draw a line continuously but push an action only when the brush is not applied anymore).
• Fix UndoRedoManager.UndoAll not calling OnBegin and OnEnd.
• Improve the History tool:
  • Gray text for redo actions instead of undo actions. (more intuitive to see undone actions in gray)
  • Selection synchronization fixed.
  • Slider aligned on items.
  • Text trimmed + tooltips when text is too long.

Update 23/07:

• Dispose IUndoableAction (if they implement IDisposable) when they are removed from undo stack. It allows to free resources kept in case the action is undone/redone.
• Dispose IUndoRedoManager when the document is closed to dispose all the IUndoableAction remaining in the stack.

[ReMinoer]

Up @tgjones

You seem to be merging pull requests again so I hope you will give it a look. :) CI failed for a SDK issue not in my hand. Re-run it will probably be enough to validate it.

[tgjones]

Thanks, looks good!