Make messages translatable.
Make function static to give local linkage.
Add support for extended selections in the StateMachineViewer.

This is mostly required in order to be able to _unselect_ an item.
Before, one could not reset the filter on a specific state.

This way, we can unselect properly _and_ also get the ability of
multi-selection which is also properly visualized. Probably quite
useful, I'm sure :)