Add support for returning item location to wxGTK wxMimeTypesManager.

This is a squash merge of gtk_mimetype branch from
https://github.com/Hanmac/wxWidgets.git with some extra minor cleanup.

Closes https://github.com/wxWidgets/wxWidgets/pull/293

include/wx/unix/mimetype.h

@@ -99,6 +99,8 @@ protected:
                        const wxArrayString& strExtensions,
                        const wxString& strDesc);
 
+    virtual wxString GetIconFromMimeType(const wxString& mime);
+
     // give it access to m_aXXX variables
     friend class WXDLLIMPEXP_FWD_BASE wxFileTypeImpl;
 };