[Trunk] Common Time and Alla Breve symbols have nominal and actual durations of numeral time signature in score creation step

1. 'New'.
2. 'Continue' until time signature step.
3. Change time signature to 12/8
4. Select either 'common time' or 'alla breve' symbols.
5. 'Done'.
6. Right-click second bar.
7. 'Measure Properties'.

Expected result: The nominal and actual durations should be 4/4 or 2/2 for 'common time' and 'alla breve' symbols respectively.
Actual result: Nominal and actual durations are 12/8.

Discussion: Marked as critical, as it may constitute corruption - the user may also be unaware.

Using MuseScore 2.0 Nightly Build (5100) - Mac 10.6.8.