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 email@example.com
Closes #903 (closed)