Add option to display and edit object position and size in tile units

[bjorn]

Requested on the forum.

This is about adding an option that changes the units used in the Properties view (and it should probably also affect the values displayed in the Objects view).

Such an option will become problematic when tile layers can have their tile size configured separately (#149), though that could be solved by either defining a "tile size" on the object layers as well or by keeping a default tile size on the map.