#121 Map part handling

[Thomas Schöps]

Map parts exist in the source code, but in practice only one part is used yet. Implement map part handling:

• Creating, deleting, merging parts
• Moving objects between parts
• Creating a new part from selected objects
• Importing selected parts from other maps
• ...

[Thomas Schöps]

One problem with the current implementation is the lack of integration with the undo system. To do:

• on undo / redo, set current map part to the part where the action happened
• on deleting parts, remove undo items related to that part (or better: create a new undo action for it)
• create an undo action for moving objects between parts
• possibly more

[Jan Dalheimer]

Haven't had much time to continue on map parts for the last few days, these are a few of the things I want to fix.

[Kai Pastor]

Undo handling remains to be done.

[Kai Pastor]

Undo handling is implemented now. So this ticket is now about to be closed.

Map parts may still need a lot of polishing and bug fixing.