Improve language selection in InstantTranslate

If a language is selected in InstantTranslate, at the moment there is the possibility in the drop down to reset the language selection.

The user feedback is, that the current mechanism is disturbing.

Please change it in the following way:

• If I select a different language in a drop-down and this would result in a language combination for which there are no engines, TMs or TermCollections
  • the other dropdown is reset to "no selection"
  • in the area of the translation a message is shown: "The selected language combination is not supported"
• If the newly selected language combination exists, the translation is triggered immediately.
• If it is easy to do: If you click a drop down, the languages that are no valid language combination in relation to the language that is selected in the other drop down, are listed in light grey instead of black. In that case on selecting an invalid combination the other drop down can stay as it is and does NOT have to be resetted to "no selection".