Collection Browser: replace "Artist" by "Album Artist" by default

CHANGES:
* Collection Browser: Artist level was renamed to Track Artist and replaced by Album
Artist by default. Various Artists item is no longer shown under Track Artist level.

Following was done:
a) shop showing "Various Artists" under "Artist" level and therefore
stop showing some tracks twice. Show "Various Artists" only under the
"Album Artist" level where it logically belongs
b) tweak the default presets to use "Album Artist" where they previously
used "Arist", except for "Album / Artist" where it doesn't make sense
c) adapt user configs treat "artist" as "album artist" for users migrating
from older Amarok releases
d) rename "Artist" level to "Track Artist" make the distinction more clear
e) use current "Artist" icon even for Album Artist level, Sentynel and
Mamarok finds the current one way too fugly

^^^ I think that the points above combined would mean nearly no
disruption in user experience and would make the interface more logical
and correct.

GUI: A set of changes to Collection Browser levels, "Artist" level was
renamed to "Track Artist", its behaviour was changed and it was replaced
by "Album Artist" by default.