Add helper AcceptCellEditControlIfShown() function

This is just another refactoring in order to avoid duplicating calls to
HideCellEditControl() and SaveEditControlValue() in several different
places.

Also call DoSaveEditControlValue() because if the editor is shown, it is
also necessarily enabled and there is no need to check for this.

include/wx/generic/grid.h

@@ -2908,6 +2908,10 @@ private:
                 );
     }
 
+    // Accept the changes in the edit control, if it's currently shown, and
+    // dismiss it.
+    void AcceptCellEditControlIfShown();
+
     // Unlike the public SaveEditControlValue(), this method doesn't check if
     // the edit control is shown, but just supposes that it is.
     void DoSaveEditControlValue();