Add 'source selection' dialog

Add a 'source selection' dialog to replace the 'enter a name' dialog.
This new dialog allows you to make new instances of pre-existing sources
so that you can add a pre-existing source to a different scene, or in to
the same scene more than once.

Also started implementing locale.

Comtemplating switching to JSON-based locale later, so we can add things
like descriptions/disambiguation, and so we can use jansson's built-in
hash table when doing the string lookup.