Simplify MCControl::setsbprop function and remove MCU_matchflags

Only two callers of setsbprop exist in (non-LEGACY_EXEC) code and they only use two properties rather than the myriad supported in the code.
Merge branch 'refactor-syntax_unicode_setsbprop' of https://github.com/runrevfraser/livecode into refactor-syntax_unicode_setsbprop