Bug 981713: Increase usability by letting translator choose source language

The TranslationEditor now contains a drop-down list where the active target locales are listed. The translator can choose a locale from the list and the translation, if any, for each phrase is shown under the source phrase.
It is also possible to hide the drop-down list via a checkbox in the settings-tab.

See issue: https://bugzilla.redhat.com/show_bug.cgi?id=981713