Add Unicode support to MCField

MCField now uses an MCStringRef to store its label.
Simplify MCField label handling

As per Mark's suggestions, simplified the label handling for MCField (the label should now never be set to nil, only the empty string).
Removed un-necessary checks for nil
Rename MCField::settext to settext_oldstring

The MCString variant of MCField::settext has been renamed to MCField::settext_oldstring. In its place, the new MCField::settext takes a StringRef.
Merge branch 'refactor-syntax_unicode_field_clean' into refactor-syntax_unicode_button

The new MCField::settext variant taking a StringRef is required.