tree: smarter artist naming and sorting [2/2]

tree: smarter artist naming and sorting [2/2]

With this path, tree view updates artist name
when better one becomes available:
- if track to be added contains some sort of pretty name
  ("artistsort", etc)
- if track to be added contains name like "The XXX",
  and current one is "XXX"

(artist identity is determined by comparing raw names
 with initial "The " striped off)

So, from now on, the logic is:
- if any of tracks of this artist contain pretty name,
  it is used
- else, if any track contains name starting with "The",
  that one is used instead (after reformatting
  to something like "XXX, The" for better sorting)
- else, raw name is used

Signed-off-by: Gregory Petrosyan <gregory.petrosyan@gmail.com>