Nautilus file operations are implemented as asynchronous jobs scheduled using
g_io_scheduler. Since g_io_scheduler has been deprecated, these operations
should be using the simpler GTask API. In order to make this change possible, a
first step would be to replace the scheduler in the helper functions called
during the jobs.

Replace g_io_scheduler_send_to_mainloop with g_main_context_invoke in helper
functions. Since g_main_context_invoke is not blocking, add a mutex and a
condition so the current thread is blocked until the operation is completed in
the main loop.

https://bugzilla.gnome.org/show_bug.cgi?id=761549