[core] Rework metadata key system

One of the initial goals of Grilo was that metadata keys should be dynamic:
besides having a pre-defined set of keys, both users and plugin's developers
should be able to add their own keys. So far, this goal was not addressed
properly.

The aim of this patch is that users/developers can add their own keys while
keeping one of the key features of current system: comparing keys must be quick
(this is how string-based keys were discarded).

In previously system, GrlKeyID were just numbers; in the new one, GrlKeyID are
pointers to GParamSpec. Being pointers, means that comparison can be done
quickly.

What are the main changes?

- No need for GRLKEYID_TO_POINTER / POINTER_TO_GRLKEYID: keys can be added
  directly to GLists.
- grl_plugin_registry_lookup_metadata_key() changed: returns the GrlKeyID with
  name specified.
- GRL_METADATA_KEY_FOO are still kept, but now they are variables instead of
  definitions.
- No need for GrlMetadataKey: GrlKeyID has the required data. Thus
  GRL_METADATA-KEY_GET_{NAME/ID/DESC} require a GrlKeyID.
- Added grl_plugin_registry_register_metadata_key (): registers a new key in
  the system. Key is built as a GParamSpec.