Update the current object in a few more cases

Now the current object (displayed in the Properties dockwidget) also
changes when clicking object layers or objects in the Objects dock,
when clicking on a terrain and when clicking on a tile. Regardless of
whether the clicked object was already selected or not.
Used Properties dock instead of the dialog for tilesets

Support was added to the PropertyBrowser for Tileset objects, obsoleting
the separate Rename Tileset action since the name can simply be edited
in the Properties dock.
Disable the properties dock for external objects

Tileset, and as a result Tile and Terrain instances, may be stored in an
external file. Currently no changes are allowed when this is the case,
in order to avoid loss of information due to the lack of automatic
saving of changes to such external files.