mouse based interactive insertion of gaps cannot be undone properly

Steps to reproduce:

1. load one of the example files into the desktop or pick a jalview applet example
2. shift+click and drag to insert a few gaps into a sequence
3. hit undo - only some of the inserted gaps are removed and the undo buffer is empty

Notes: only mouse based editing is broken - keyboard insert/undo is working fine. Bug behaviour suggests the code generating the undo item is not getting all the edit events.