Use "pair of ints" type for wxGridBagSizer size and position in XRC

Introduce a new type for XRC values imaginatively called just "pair of
integers" which can be used for values not expressed in pixels and hence for
which it doesn't make sense to use dialog units nor to scale them by the DPI.

Use this new type for wxGridBagSizer position and span elements to prevent
them from being changed when using higher than normal DPI.

Closes #17592.

include/wx/xrc/xmlres.h

@@ -544,6 +544,9 @@ public:
     wxCoord GetDimension(const wxString& param, wxCoord defaultv = 0,
                          wxWindow *windowToUse = NULL) wxOVERRIDE;
 
+    // Gets a size which is not expressed in pixels, so not in dialog units.
+    wxSize GetPairInts(const wxString& param) wxOVERRIDE;
+
     // Gets a direction, complains if the value is invalid.
     wxDirection GetDirection(const wxString& param, wxDirection dirDefault = wxLEFT) wxOVERRIDE;