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

The references are not updated directly if another translator changes a translation. Since two languages communicates on different channels, this feature would make the channels very busy. According to Sean Flanigan he has "been thinking that we need to make the channels narrower, and I think this feature will make it more urgent - instead of a channel for project+version+language, perhaps we need channels for individual document+language." 

Based on this I have not implemented a listener for TransUnitUpdatedEvents in the new drop-down list.
The references are updated if you switch language though.