Fix exporting clipboard data to primary selection in wxGTK.

Honour the requested selection in our selection handler instead of always
returning the default one resulting in wrong data being pasted when using
primary selection (e.g. middle clicking).

Closes #12947.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67229 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/clipbrd.h

@@ -65,8 +65,8 @@ public:
     // get our clipboard item (depending on m_usePrimary value)
     GdkAtom GTKGetClipboardAtom() const;
 
-    // get the data object currently being used
-    wxDataObject *GTKGetDataObject() { return Data(); }
+    // get the data object currently being requested
+    wxDataObject *GTKGetDataObject( GdkAtom atom );
 
     // clear the data for the given clipboard kind
     void GTKClearData(Kind kind);