Ignore special tracks for completeness

This adds an option to ignore some track types for determining the completeness of an album. This makes it easier for a user to see completed albums and ignore tracks she doesn't care about. Currently the user can choose to ignore:

* Video tracks
* Data tracks
* Pregap tracks
* Silent tracks

In addition this PR adds some more changes to simplify the work with those tracks:

1. Video and data tracks get a different icon instead of the default musical note icon (in case a track is both a data and a video track the video icon gets precedence)
2. Adds a `$is_complete()` tagger script function
3. Sets a `~video` tagger script variable for video tracks
4. Ignored tracks are displayed grayed out, so they are still available for tagging but not as prominent in the listing

This PR will fix [PICARD-514](http://tickets.musicbrainz.org/browse/PICARD-514) and [PICARD-652](http://tickets.musicbrainz.org/browse/PICARD-652)

This PR **is not ready for merging** due to the new icons. I have added icons from the Tango icon set for the track types because this is the icon set the current note icon is taken from. But I think the icons are a) really ugly and b) make the UI way too noisy. But since I am not very talented in designing icons and I am not sure when I will find the time to look for nicer icons with an acceptable license I just publish the code here and **ask you all for help** :)

Some notes about the `$is_complete()` function: This is somewhat similar to the `$isperfectalbum` function @Sophist-UK proposed in [PICARD-637](http://tickets.musicbrainz.org/browse/PICARD-637). But I deliberately decided not to add the special logic of having only one matched file per track. I think this is a different change which should be discussed separately. For me it was important that `$is_complete()` and the visual representation of the completeness are identical.
I would just write `return (self.ignored_for_completeness() or self.num_linked_files == 1)`