Allow registering of custom wxWebView backends.

Add wxWebViewFactory as an abstract factory to provide backend creation. Remove old factory methods using wxWebViewBackend enum in favour of the new wxString based method.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73369 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775