Change the uri of existing media or color paint with double-click

When double-clicking a paint QListWidgetItem, the uri of that paint can be
altered.

A new signal is sent to ColorMapper and TextureMapper objects so that they can
update heir color and texture paints respectively. Mappings of the old paint are
transferred to the new paint.

TODO: the active mapping may be lost in some cases, needs some more testing.
Merge branch 'develop' of github.com:mapmapteam/mapmap into develop