egg-task-cache: handle the cancellation in an idle callback

The caller's GTask was created in an earlier iteration of the
GMainLoop. Hence, the g_task_return_error_if_cancelled will complete
immediately, and there is a possibility that the GTask might be
destroyed before returning to the GMainLoop. If that happens, it will
deadlock because it would be disconnecting from the GCancellable
from the GCancellable::cancelled handler.

It is also not safe to avoid disconnecting from the GCancellable, in
case it was cancelled, because, if reset, it can emit the signal a
second time and we will be accessing invalid memory.

Hence the safest option is to handle the cancellation in an idle
callback and immediately returning to the GMainLoop in the "cancelled"
handler.

https://bugzilla.gnome.org/show_bug.cgi?id=779660