ui: context/vocabulary revamp: fixing showstoppers

Squashed commit of the following:

commit 97c3e3c7d867939ba2bf899a2f0c9f0e80f01760
Author: Max F. Albrecht <1@178.is>
Date:   Tue Jun 3 14:41:16 2014 +0200

    fix the test #2

commit e9a72336092682a04eb06ebfff16a6c0d4b5586a
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 2 16:48:51 2014 +0200

    fix the test

commit aa394166f9bac1fcbf5e6d864a6f7949abbb2aa4
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 2 15:21:01 2014 +0200

    wording

commit 0efad30b9e825b1ff456d74371c0c9b1e84a5e3e
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 2 15:20:47 2014 +0200

    ask for confirmation before removing context from set

commit 7cd61aa7741bc4709af8be8ac2f3a9188ce92879
Author: Max F. Albrecht <1@178.is>
Date:   Mon Jun 2 15:20:03 2014 +0200

    also guard media_set controller agains contexts with empty keys