GROOVIE: Change the movie speed option to a number

This simplifies the movie speed option and changes it to a number
(0 - 2), so that it's easier to understand its usage.

Also, this ensures that all per-engine/per-game options are either
booleans or numbers, so they can be represented in the launcher's
GUI via checkboxes (which is already implemented) and via scroll
bars (which can be done in the future). The previous string setting
for T7G's movies could not be represented in our GUI