Moved the drop-down list for selecting language to the options panel and removed the checkbox.
Fixed typo in ReferenceVisibleEvent.java.

Remaining issues:
* The options panel do not scroll.
* The persisted reference language is not selected when opening the webtrans.
The persisted referenceLang is now show when loading the editor.
Renamed TransUnitChangeSourceLang to ChangeReferenceLang.
Added a DisclosurePanel as a wrapper around the referenceLabel so it is only shown for the selected row, padding and styling needs to be added.