Remove g_type_init() calls

But call it in testing_util_init() if GLib < 2.35.0.
Make calling testing_init() required

Also encapsulate the order in which {g,gtk}_test_init()
must be called.
Fix building python plugin loaders
Store icons as a GIcon when possible

This allows stock id icons to work.
Fix a few typos
Do not abort if a warning occurs during testing_show_widget()