Add undo support

[gavanderhoorn]

As per subject. The application should allow the user to undo any (possibly destructive) action he/she has performed.

Some considerations:

• unlimited undo or only since last save?
• should only edit actions be undoable (ie: changing values of properties) or also link/joint addition & removal?

[gavanderhoorn]

This will probably require (quite) some refactoring of the application: at the very least all user-initiated actions would probably need to be reworked into some variant of the Command pattern.

There appears to be (some) support for this in Qt already. See Overview of Qt's Undo Framework and Undo Framework Example (5+ specific though). Whether this particular framework is usable given the constraints of this application needs to be determined.