Add Unicode support to MCGraphic

MCGraphic now uses an MCStringRef to store its label. Wrapper functions MCFontMeasureText and MCFontDrawText were added - these take an MCStringRef rather than a {const char * p_text, size_t p_text_length, bool is_unicode) triple. The wrappers will need modification when MCScreenDC can take an MCStringRef in its corresponding methods.
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).
Merge branch 'refactor-syntax_unicode_field' into refactor-syntax_unicode

Need MCField to support Unicode/StringRefs as MCButton::entry is of this type and is heavily referenced.
Add support for properties of type BinaryString

The MCProperyObjectThunk{Get,Set}BinaryString macros have been added to allow a BinaryString using the MCDataRef type to be defined as a property using DEFINE_*_PROPERTY macros.