Add constant for coverart archive host.

Signed-off-by: Philipp Wolfer <ph.wolfer@gmail.com>
Add option to limit browser integration to localhost.

This will make Picard's browser integration to listen on localhost only, as listening on the public interface is a possible security and privacy concern. There is still an option to enable listening on the external interface to allow for special requirements such as using the Picard Barcode Scanner android app.
Apply changes to browser integration only on save.

Fixes the case where the user disables the browser integration checkbox but does not save the settings. This lead to the browser integration being disabled while the option was still enabled.
Added a constant for the coverart acrhive port.
Added a constant for the acoustid port.
Add an option in Advanced>Interface to set starting directory.

By default, Picard remembers last directory used and restores it on start,
but this isn't not always desirable, one may prefer to start on the same directory
each time.
This patch allows one to change default behavior and set a starting directory.
Reduce code redundancy and use starting directory for Add folder action
Add action 'Set as starting directory' to file browser context menu
Track featured artists don't indicate compilation

This fix only uses primary track artist to determine whether an album is
a compilation. Featured artists no longer influence this.
Remove unnecessary comment
Compilation if set in MB releasegroup releasetype
Change the text for selecting the starting directory

It was "Open this directory when starting:" which is not what's
happening: the selected directory would only be visible if the user is
using the file browser, but not if he's only using the "Add folder"
button.
Add the starting directory to NEWS.txt
Regenerate picard.pot
Fix save/restore of metadata box column sizes.

In some cases, mostly after maximizing/restoring window a few times,
metadata table columns were smaller and smaller due to wrong calculations.
Also when window was resized, weird things were appearing with those columns.
This patch is handling those events much better (but still not perfect).
Horizontal scroll was changed to be per pixel instead of per column.

It tries to handle scrollbar appeareance/disappearance, maximize/minimize cycles, restart cycles, elegantly.
New AcoustID API key
Remove an unused import
browser_lookup: If the item is a DataObject, use its already known id

DataObject objects are initialized with an mbid, we can use that to perform the
browser lookup.

Fixes PIACRD_86
Remove superfluous variables

Those are write-only variables.
replaygain: Use a defaultdict in split_files_by_type