Adding IsSolid to wxColour

Under macOS colors can be patterns, then accessors for RGB values are useless, IsSolid returns true if the color can be expressed in RGB values at all.

include/wx/colour.h

@@ -118,6 +118,9 @@ public:
     virtual ChannelType Blue() const = 0;
     virtual ChannelType Alpha() const
         { return wxALPHA_OPAQUE ; }
+    
+    virtual bool IsSolid() const
+        { return true; }
 
     // implemented in colourcmn.cpp
     virtual wxString GetAsString(long flags = wxC2S_NAME | wxC2S_CSS_SYNTAX) const;