wxGenericProgressDialog needs to have an active event loop in order to repaint
itself and process clicks on its buttons but it's more helpful to create a
temporary event loop if there is no currently active one instead of just
asserting. In particular, this allows to use wxProgressDialog from overridden
wxApp::OnInit().
Add temporary event loop creation and remove the now unnecessary asserts
verifying that there is an active event loop as there always will be one.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65499 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
7ad8a38ae5