Add native RTF support to wxTextCtrl (macOS)

[Blake-Madden]

This adds:

• GetRTFValue() and SetRTFValue() functions to get and set raw RTF
• Changes wxTEXT_TYPE_ANY into a wxTextCtrlFileType enum that includes a new wxTEXT_TYPE_RTF option
• LoadFile() and SaveFile() now use their fileType parameter to save as either text or RTF (if supported)
• Demonstrates these new feature in the "text" sample
• Simple unit test

P.S. An MSW implementation for the RICHEDIT2 control will be forthcoming in another PR if this is accepted.