Change MCSystemPlaySound to take a StringRef

Both iOS and Android platforms changed.
Change MCSystemPlayVideo to take a StringRef

Both Android and iOS platforms have been modified.
Updated MCSystemGetPlayingSound to return a StringRef

The s_sound_file global variable is now assigned an empty string when no sound is playing rather than being nil.
Restore accidentally removed behaviour in MCMultimediaEvalSound

The pre-refactor behaviour was to return the string "done" when no sound is playing; this was accidentally removed.
Revert edits to cmdse.cpp and funcs.cpp

Accidental edits made to code within an #ifdef LEGACY_EXEC block.
Use MCStringBeginsWithCString where appropriate

Replaced calls to MCStringBeginsWith using a C string with calls to the more appropriate function.
Fix an incorrect function signature

The extern declaration for MCSystemGetPlayingSound in MCMultimediaEvalSound had the wrong signature.
Add MCSystemSound{Initialize,Finalize} functions

Both Android and iOS now call these functions at startup/shutdown.
Merge pull request #174 from runrevfraser/refactor-syntax_unicode_mblmedia

Refactor syntax unicode mblmedia