Add undo/redo functionality for the tree

[robotframework-ride]

Originally submitted to Google Code by andreas.brain on 20 Feb 2012

The 'sorting keywords' functionality that was added in 0.41 provides the possibility for undo/redo. The according commands are pushed to the stacks, but the entries in the menu stay disabled.

From IRC:

mkorpela: There are multiple undo stacks seems like commands started from the tree aren't currently undoable if the code works on your gitrepo we could take a look at it tommorrow -- and we could also add an issue about making undo work from the tree