Better handling for asserts in non-main threads.

Better handling for asserts in non-main threads.

Don't call wxTrap() when an assert in a non-main thread fails. As asserts are
now always enabled by default, this is not a good idea. Instead just show the
full details about the assert failure using wxMessageOutputDebug under the
platforms without MT-safe message box function (i.e. everything but MSW
currently).

Add a possibility to test an assert happening in non-main thread to the except
sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62842 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775