[2.0] discoverer: use GTask etc.

Submitted by an unknown user

Link to original bug (#669018)

Description

<pbor> Uraeus: does gst have a lot of API with GErrors passed as signal arguments?
<pbor> it's a pattern that is usually avoided in gtk etc
if it's not common maybe now that you are breaking the api it is a good time to avoid it
pbor, we got it in discoverer (which is where I get this crash https://bugzilla.gnome.org/show_bug.cgi?id=668862
<tomeu> opportunity to make the API more consistent :)
<pbor> Uraeus: the pattern used in other places is that you get a signal that the an operation is complete and then you call finish(&err) to know if it failed and to retrieve the error

Version: 2.x