Abstract method to determine if track number has changed.

This is a little more accurate than the previous method (check if track
is in index or medium_index) by looking at the `per_disc_numbering`
setting and comparing the index or medium index accordingly.

It's also a little more accurate in the display output by diffing the
combined `disc-track` to `medium-medium_index` (if using per disc
numbering) and intelligently colorizing the either the whole track
number or just the suffix.
Don't colorise the "()" around partial match text. For consistency (and a little easier to read?)
Indent album title and artist when there are no changes for consistency with when there are (and easier to read?)
Display album ambiguation on info line (after similarity and source). Colorise disambiguation light grey for subtle distinction from album text.
Colorize benign track index changes (to/from per disc numbering) in yellow.
No penalty for track index changes to/from per disc numbering.
Colorize parenthesis around partial match message.
Display each part of info line in parenthesis. Source and disambiguation info are conditional.
Display constant data first (similarity) followed by optional data (partial match, disambiguation). Use consistent and colorized parenthesis for optional data.
Merge pull request #284 from mrmachine/ui-improvements

UI improvements and consistency. Colorisation, indentation, position by importance, and highlight benign changes.

1. Consistently colorise parenthesis surrounding colorised text that is conditional (doesn't always appear) so that it stand out more. This includes the album info data source (when it's not MusicBrainz), the album disambiguation (sometimes there is none), and the partial match message.

2. Indent the album title when there isn't a change. Makes it stand out more from the file path(s) above and info line below, and is consistent with the indentation when there is an album or artist change.

3. When listing releases, colorise album disambiguation in light gray to distinguish it more clearly from the album title. Light gray seems like a good colour because it is subtle, and disambiguation is supplementary information (not green/success, red/error or yellow/warning).

4. Also when listing releases, display similarity before partial message and disambiguation last. This is in order of importance, and also similarity is always present while the others are conditional, and finally it is often the case where the album title is the same and only the disambiguation is different. Using this order keeps things lined up in this case and easier to visually compare similarity and disambiguation text.

5. Colorise benign track index changes (to/from per disc numbering) in yellow, without applying a match penalty. We should still display these because even though the track number is still correct/equivalent, beets will apply a change if the new tags are applied.

6. Don't display source unless it's not MusicBrainz. By default beets will only find matches in the MusicBrainz database. Only if a plugin is enabled can a different source be used, so not everybody needs to see this all the time.