Each project should contain separate undo stack

[vt4a2h]

In the current implementation we have only one undo stack for all projects. This potentially lead to inconsistency and memory leaks.

Acceptance criteria:

1. Separate undo stack should be created per project
2. When active project has changed, current command stack should be changed as well
3. When project is closed, its undo stack should be dropped as well
4. Command for changing active project shouldn't be added to any undo stack at all
5. Deactivated project should retain its modified state (and all other states)