[Trunk] Selecting common time or alla breve symbol during score creation should grey out numerals

1. 'New...'.
2. Click 'Continue' until 'Create Time Signature' step.
3. Select either a common time or alla breve symbol.

Desired result: The numerals of the other time signature (not common time or alla breve) should be greyed out.

Discussion: It might be a separate report, but having multiple options causes confusion. If you create the score, not only can you have either symbol, but also numeral and actual durations of 12/8. Common time and alla breve symbols can only be 4/4 and 2/2 respectively.

Using MuseScore 2.0 Nightly Build (5091) - Mac 10.6.8.