Fix repositioning of active property editor in wxPG

When there is open an editor for some property and in the same time
wxPropertyGrid layout is changed (due to the adding or removing a property,
sorting), it is necessary to recalculate the actual position of the active
editor to display it in the cell dedicated for the edited property. Under
some platforms the position of the edit control is shifted within the cell
and we have to take this shift into account in repositioning process.
Because actual value of the shift depends on the platform and on
the particular control, it is convenient to determine actual shift when
the editor is created and use this value whenever repositioning is done.

Close #17912.

include/wx/propgrid/propgrid.h

@@ -1465,6 +1465,9 @@ protected:
     // wxWindow pointers to editor control(s).
     wxWindow            *m_wndEditor;
     wxWindow            *m_wndEditor2;
+    // Actual positions of the editors within the cell.
+    wxPoint             m_wndEditorPosRel;
+    wxPoint             m_wndEditor2PosRel;
 
     wxBitmap            *m_doubleBuffer;
 
@@ -1644,8 +1647,9 @@ protected:
     // pointer to property that has mouse hovering
     wxPGProperty*       m_propHover;
 
-    // Active label editor
+    // Active label editor and its actual position within the cell
     wxTextCtrl*         m_labelEditor;
+    wxPoint             m_labelEditorPosRel;
 
     // For which property the label editor is active
     wxPGProperty*       m_labelEditorProperty;