Remove static MainLoop, MainContext, and some GSource bindings

glib's MainLoop and MainContext are fully introspectable these days, so remove
our static bindings. This reduces our code, as well enables GLib API which
hasn't been available through the static bindings before.

This also requires dropping our custom static types for GLib Source, Timeout,
and Idle. The latter two work fine with introspection and just need tiny
overrides for a backwards compatible API. g_source_new() is not introspectable,
though, so we need to keep our static wrappers for that. Move them from
gi/_glib/pygsource.c to gi/pygi-source.c, so that it can use the GI API.

Note that gi/_glib/pygsource.[hc] is still required for the static PollFD type
which is used by the static IOChannel binding. Once the latter goes away,
PollFD can be dropped as well.

https://bugzilla.gnome.org/show_bug.cgi?id=686443