Clear grid selection more consistently prior clear operations; improved Clear() tests (fixes #10497)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58971 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/propgrid/manager.cpp

@@ -657,6 +657,8 @@ wxPropertyGridPageState* wxPropertyGridManager::GetPageState( int page ) const
 
 void wxPropertyGridManager::Clear()
 {
+    m_pPropGrid->ClearSelection(false);
+
     m_pPropGrid->Freeze();
 
     int i;