add editor tool to set a new origin in the map

[Sigra]

Add an editor tool to set a new origin in the map. This will make it possible to get an island properly centered, instead of divided in 4 corners. See the map download section with previews at [http://wl.widelands.org/maps/]. Put a button that enables the tool in the map options dialog.

The attached patch implements this (and some code refactoring for ui_button).

[Sigra (sigra)]

The file reorigin-1.diff was added: patch

[Sigra (sigra)]

Committed in revision 3778.