Rework of full resolution

[Guillaume Emont <gemont igalia com>]

This patchset reworks full resolution in the following ways:
 - _key_depends() is deprecated by a new _may_resolve() that provides more
   information and cleaner semantics, fixing bug #640667 [1]
 - the functionality of grl_metadata_source_setup_full_resolution_mode() is
   split into two parts, grl_metadata_source_expand_operation_keys() and
   grl_metadata_source_get_additional_sources(). One being called before the
   operation, and the other one after.

I have started work on proper unit testing of this, based on test sources I had
already started [2], but as it is a very lengthy task, I have decided to
postpone it for now. Instead, I ran some tests with grilo-test-ui trying to
cover most of the common cases that happen with the current metadata providers
we have, and things seem to run smoothly.

It now gives real usefulness to plug-ins such as lastfm, gravatar or
local-metadata, since they are now called and able to provide information that
the media source claimed it could get but didn't manage to get (common with
e.g. upnp or tracker).

Guij

[1] https://bugzilla.gnome.org/show_bug.cgi?id=640667
[2] http://www.gitorious.org/~guijemont/grilo/guijemont-sandbox/commits/test_sources