OSX use fontAttributes for better emulation of bold fonts (#501)

* using fontAttributes dictionary allows for better emulation of bold typefaces, also switch to using color from context, this allows keeping the same dict throughout, as a side effect fix font caching

* reverting in order to maintain crossplatform compatibility

* applying review suggestions

* applying review suggestions

include/wx/osx/font.h

@@ -144,6 +144,7 @@ public:
 #endif
 
     CTFontRef OSXGetCTFont() const;
+    CFDictionaryRef OSXGetCTFontAttributes() const;
 
 #if wxOSX_USE_COCOA
     WX_NSFont OSXGetNSFont() const;