Allow multiple classes to be in edit mode

[Matthias Schoettle]

Currently, the implementation of StructuralDiagramView doesn't allow multiple classes to be in edit mode. If we want true multi-user support we need to allow multiple classes to be in edit mode.

One case where this is currently not possible (steps to reproduce):

• Tap-and-hold on two classes to put them into edit mode
• Tap the background
• Only the edit mode of the last class is disabled

[Matthias Schoettle]

Fixed through the following commits:

• 85155a5: Multiple classes can be put into edit mode.
• 1c6a2c3: Added "edit done" button to go out of edit mode.
• fb3a6ea and ac032dc: Added "edit done" to handlers.