Allow drawing collision shapes on tiles

This change associates an ObjectGroup with each Tile on-demand and adds
a new "Collision Editor" dock that allows editing this object group for
the selected tile.

Closes #552
Made the ClipboardManager into a singleton

This makes sense since there is never a need for two clipboard managers,
and I want to access it from multiple places.
Fixed start drag distance when moving objects

See also commit d385b3e85b79546c2c5ccabc0c3d4fecdcd97fba.
WIP: Tile Collision Editor