If tasks block waiting for other tasks to complete then the system can
end up starved for threads. Avoid this by bumping up max-threads in
that case.
This also reverts 7b1f8c58 and reverts max-threads for GTask's
GThreadPool back to 10.
https://bugzilla.gnome.org/show_bug.cgi?id=687223
Very many testcases, some GLib tools (resource compiler, etc) and
GApplication were calling g_type_init().
Remove those uses, as they are no longer required.
https://bugzilla.gnome.org/show_bug.cgi?id=686161
GTask is a replacement for GSimpleAsyncResult and GIOScheduler, that
also allows for making cancellable wrappers around non-cancellable
functions (as in GThreadedResolver).
https://bugzilla.gnome.org/show_bug.cgi?id=661767