Move ID3Metadata functionality to new_metadata property

The problem with ID3Metadata is that it changes the behavior of
Metadata methods in undesirable ways (see PR #273).

This adds a new_metadata property to File classes which returns a copy
with the values modified as they'd be displayed/saved. This should more
composable and less error-prone than overriding Metadata methods across
the board.
Regenerate attributes.py
Partially update translations.

> 95% only
Regenerate picard.pot
Move swapprefix to Picard proper...

... and add similar delprefix
Integrate a couple more plugins ...

eq2 and ne2 by Brian Schweitzer.
Add NEWS.txt item.
Extend eq / ne reather than new eq2 / ne2
Revert reuse of eq/ne and rename again
Adopt simplification proposed by mineo
Fix typo in function documentation.
Use regex, fix typos.
Add tests
Adjust NEWS.txt
Add eq_all, ne_any as requested by Freso
Remove unnecessary for loops (bitmap)
Fix terminology inconsistency ...

... created in #238
Merge pull request #281 from mwiencek/kill-id3metadata

Move ID3Metadata functionality to new_metadata property
Merge pull request #292 from Sophist-UK/sophist_fix_inconsistency_in_#238

Fix terminology inconsistency ...
Add all .py files from contrib/plugins to executable