Save specified, not actual, col width in wxPersistentDataViewCtrl

This is more correct as saving the current width of the last column
would prevent the user from shrinking it under the last automatically
set size, i.e. the UI behaviour would change after restarting the
program, which shouldn't be the case.

Doing this required making WXGetManuallySetWidth(), which previously
existed in the generic version only, available in all ports, so do it
and also rename it to WXGetSpecifiedWidth() in the process, as this
seems a somewhat better name (it doesn't have to be manually specified,
i.e. it could also be done by the program itself or even implicitly by
wxPersistentDataViewCtrl).

Don't make this function public, at least for now, because it's not
clear how could it be useful and it might still need to be changed to
behave differently in the other ports.

include/wx/persist/dataview.h

@@ -61,7 +61,15 @@ public:
             SaveValue(columnPrefix + wxPERSIST_DVC_HIDDEN, column->IsHidden());
             SaveValue(columnPrefix + wxPERSIST_DVC_POS,
                       control->GetColumnPosition(column));
-            SaveValue(columnPrefix + wxPERSIST_DVC_WIDTH, column->GetWidth());
+
+            // We take special care to save only the specified width instead of
+            // the currently used one. Usually they're one and the same, but
+            // they can be different for the last column, whose size can be
+            // greater than specified, as it's always expanded to fill the
+            // entire control width.
+            const int width = column->WXGetSpecifiedWidth();
+            if ( width > 0 )
+                SaveValue(columnPrefix + wxPERSIST_DVC_WIDTH, width);
 
             // Check if this column is the current sort key.
             if ( column->IsSortKey() )