Create temporary wxEventLoop in wxGenericProgressDialog if needed.

Create temporary wxEventLoop in wxGenericProgressDialog if needed.

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