Add %_trackindex% to fix Picard-527 ...

... for e.g. Vinyl whose tracks are numbered A1, A2, ..., B1, B2, ...
Add NEWS.txt
Remove old/* plugins.

It's been almost 3 years since they were moved to the old/ folder, and
apparently they've not been needed enough by anyone to get an update
to current plugin API. If ever they're deemed useful again, they can
be pulled back via Git and updated.
Remove `open_in_gui.py` plugin.

The functionality is included in Picard's core now, so there is no
need for a plugin to do it as well.
Strip CR as well as LF to avoid errors on Windows
Merge pull request #255 from Freso/cleanup-contrib-plugins

Clean up contrib plugins
Use Universal Newline Support instead.
Merge pull request #257 from Sophist-UK/sophist_fix_test_bytes2human

Fix test bytes2human on Windows
No known instances of this problem but ...

... if plugin was located in picard.plugins_fake.smurf then module would
be set to "fake.smurf" rather than None.
Merge pull request #258 from Sophist-UK/sophist_fix_minor_bugette_in_plugins

Possible minor bugette in plugin.py
Switch to ~musicbrainz_tracknumber
Merge pull request #251 from Sophist-UK/sophist_add_trackid_variable_Picard-527

Add %_musicbrainz_tracknumber% to fix Picard-527 ...