gmain: Officially deprecate g_main_context_wait()

It’s been de-facto deprecated for a long time, due to emitting a
critical warning when used in a non-internal context. Make that official
in the documentation and with a deprecation annotation.

Split the implementation into an internal helper and an external
wrapper, so the two remaining internal uses don’t emit deprecation
warnings.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#903
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