Allow clearing wxPG from within wxPG event handlers.

If wxPG::Clear is called from within event handler then it is not possible to delete all property items directly because some vital internal wxPG data are still in use. In this case it is necessary to put all items on the list for deferred deletion in wxPG idle state.

Closes #16222.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76890 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775