Only allow correct value types in properties fields

[meh2481]

E.g. timebugprobability should only be allowed to be a numeric value between 0 and 1.0

This should be done at the UI step, not at a separate error-checking step, so rather than throwing a warning dialog up when you try to play the level, it'll just prevent you from making the mistake in the first place.

[meh2481]

Apparently timebugprobability also allows "false" as a valid value

[meh2481]

Addin tab, ocd field, type property only allows 3 possible values, entering it manually is strange.