Refactor generic wxDataViewColumn to use DoGetEffectiveWidth()

This function will be used for m_manuallySetWidth too, in addition to
m_width, in the next commit.

No changes yet in this one.

src/generic/datavgen.cpp

@@ -188,9 +188,9 @@ void wxDataViewColumn::Init(int width, wxAlignment align, int flags)
     m_sortAscending = true;
 }
 
-int wxDataViewColumn::GetWidth() const
+int wxDataViewColumn::DoGetEffectiveWidth(int width) const
 {
-    switch ( m_width )
+    switch ( width )
     {
         case wxCOL_WIDTH_DEFAULT:
             return wxWindow::FromDIP(wxDVC_DEFAULT_WIDTH, m_owner);
@@ -200,10 +200,15 @@ int wxDataViewColumn::GetWidth() const
             return m_owner->GetBestColumnWidth(m_owner->GetColumnIndex(this));
 
         default:
-            return m_width;
+            return width;
     }
 }
 
+int wxDataViewColumn::GetWidth() const
+{
+    return DoGetEffectiveWidth(m_width);
+}
+
 void wxDataViewColumn::WXOnResize(int width)
 {
     m_width =