Partially implement wxTextCtrl::GetStyle() in wxGTK.

Support retrieving the (both foreground and background) colours for the given
position.

Closes #11281.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62262 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/textctrl.h

@@ -63,6 +63,7 @@ public:
     virtual void DiscardEdits();
 
     virtual bool SetStyle(long start, long end, const wxTextAttr& style);
+    virtual bool GetStyle(long position, wxTextAttr& style);
 
     // translate between the position (which is just an index in the text ctrl
     // considering all its contents as a single strings) and (x, y) coordinates