Better undo/redo and various other improvements

• Rewrote undo/redo functionality:
  • added support for bullet lists.
  • kept track of caret position.
  • simplified code.
• Added option to tune the note autosave time interval (does not affect automatic saving when closing Eclipse).
• Fixed bug that would cause Eclipse to hang when adding a new note to an empty notepad.
• Various code enhancements.