Bug 618737 - "dispatch to context" functionality

Bug 618737 - "dispatch to context" functionality

Adds a new function g_main_context_invoke() (and _full() variant).

This function takes a main context, a function and a user_data.  If the
main context is already acquired in the current thread, the function is
invoked directly.  If the main context is the default main context of
the current thread and it can be acquired then the function is invoked
directly while the context is owned.  Otherwise, the function is
scheduled as an idle on the context.