Make the undo and redo stacks non-static members of AssFile, making it
theoretically possible to have multiple open AssFiles with working undo.
Slightly improve tracking of whether the file is modified: saving,
making a change, then undoing the change now results in the file being
shown as unmodified as with most programs with undo.
Add basic undo coalescing support.
Originally committed to SVN as r4667.