Accept only real idle events in wxPropertyGrid::OnIdle()

Fake idle events generated e.g. by calling wxYield from within the event handler should be skipped to avoid problems.

See #16617

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78028 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775