refactor undo/redo system to abstract view model

the undo and redo history is part of the model for a jalview session, but currently resides in the gui implementation model (jalview.*gui.AlignFrame.undoList/etc). Moving it to the model will allow undo/redo support to be easily implemented for out-of-gui operations.

Need to refactor the stacks, addUndoItem and related functions to the model, and ensure that the gui implementation's 'alignmentUpdated' method is called whenever the undo/redo stacks are touched, or alternately, introduce a new light-weight method/event analogous to the 'AlignFrame.updateEditMenuBar()' method which handles any change in the undo/redo stacks.