Fix not resorting generic wxDVC when model sort order changes

We can't skip resorting wxDataViewCtrl contents when the sort order is
defined by the model class, as it can change at any moment, so restrict
this optimization to the cases when we're sorting by the value of some
column.

Closes https://github.com/wxWidgets/wxWidgets/pull/2153

src/generic/datavgen.cpp

@@ -110,6 +110,8 @@ public:
 
     bool IsNone() const { return m_column == SortColumn_None; }
 
+    bool UsesColumn() const { return m_column >= 0; }
+
     int GetColumn() const { return m_column; }
     bool IsAscending() const { return m_ascending; }
 
@@ -1888,9 +1890,10 @@ void wxDataViewTreeNode::Resort(wxDataViewMainWindow* window)
     {
         wxDataViewTreeNodes& nodes = m_branchData->children;
 
-        // Only sort the children if they aren't already sorted by the wanted
-        // criteria.
-        if ( m_branchData->sortOrder != sortOrder )
+        // When sorting by column value, we can skip resorting entirely if the
+        // same sort order was used previously. However we can't do this when
+        // using model-specific sort order, which can change at any time.
+        if ( m_branchData->sortOrder != sortOrder || !sortOrder.UsesColumn() )
         {
             std::sort(m_branchData->children.begin(),
                       m_branchData->children.end(),