Also implement wxDataViewColumn::UnsetAsSortKey() for wxGTK

Allow to reset the sort order in native GTK+ implementation too.

See #18176.

docs/changes.txt

@@ -111,6 +111,7 @@ All (GUI):
 wxGTK:
 
 - Implement wxTextCtrl::HitTest() for single line controls.
+- Implement wxDataViewColumn::UnsetAsSortKey().
 - Fix not showing wxInfoBar with GTK+ 3 < 3.22.29.
 - Fix the build with glib < 2.32 (e.g. CentOS 6).
 

include/wx/gtk/dataview.h

@@ -43,6 +43,7 @@ public:
 
     virtual void SetSortable( bool sortable ) wxOVERRIDE;
     virtual void SetSortOrder( bool ascending ) wxOVERRIDE;
+    virtual void UnsetAsSortKey() wxOVERRIDE;
 
     virtual void SetResizeable( bool resizable ) wxOVERRIDE;
     virtual void SetHidden( bool hidden ) wxOVERRIDE;

src/gtk/dataview.cpp

@@ -3345,6 +3345,17 @@ void wxDataViewColumn::SetSortOrder( bool ascending )
     internal->SetDataViewSortColumn(this);
 }
 
+void wxDataViewColumn::UnsetAsSortKey()
+{
+    GtkTreeViewColumn *column = GTK_TREE_VIEW_COLUMN(m_column);
+
+    gtk_tree_view_column_set_sort_indicator( column, FALSE );
+
+    wxDataViewCtrlInternal* internal = m_owner->GtkGetInternal();
+    internal->SetSortColumn(-1);
+    internal->SetDataViewSortColumn(NULL);
+}
+
 bool wxDataViewColumn::IsSortOrderAscending() const
 {
     GtkTreeViewColumn *column = GTK_TREE_VIEW_COLUMN(m_column);