Make Picard search box smart enough to detect mbids.

If a release mbid is given (search type = Album) it will try to load it as
when using web browser tagger link.

If string looks like a known entity (artist, release-group, label, track)
it will open browser on corresponding url.

Examples:
Select box on Album + "abb7caf1-ac93-48e1-a066-60caad07e1a7" -> load release (http://musicbrainz.org/release/abb7caf1-ac93-48e1-a066-60caad07e1a7)

Select box on anything + "http://musicbrainz.org/artist/c19ff12b-058f-44a8-b245-b0efb4752925" -> open browser on Artist page
"ARTIST := c19ff12b-058f-44a8-b245-b0efb4752925 !!" -> open Artist page
Complete the list of known entities.