Replace nautilus_window_set_initiated_unmount() with an initiated-unmount signal

Replace nautilus_window_set_initiated_unmount() with an initiated-unmount signal

This signal just carries a boolean initiated_unmount argument, which
is meant to say True when the unmount/eject starts, and False when
the operation finishes.

We may want to rename all of this to unmount_started / stopped, etc.
I don't know if the caller needs to know about the mount_op in question,
or if it can be inferred from what the caller knows to be the current
shown location on the GtkPlacesSidebar.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>