Improve webservice prioritisation.

Ctrl-F5 Refresh - Priority/Important
Redirect - Priority/Important
Saves/submissions - Priority/Not-Important
Everything else - Normal
Tweak when we prioritise and don't cache
Space moved

As per bitmap's review comments, I have relocated the errant space from
one file to the other.

Damn pesky spaces - always sneaking off and repositioning themselves
where you don't want 'em.
Address Fresno comment

Put important back to True so that this is LIFO on the priority queue.
Merge pull request #166 from Sophist-UK/sophist_improve_xmls_prioritisation

Improve webservice prioritisation.
Add more info to release name in other releases list if needed.

Context menu listing other releases is using main information from release,
often it is enough, but sometimes two or more items in the list are exactly
the same, user have to select each one to determine which is matching for him.

This patch is improving a bit the situation by adding packaging, barcode, or even
disambiguation comment if needed.
As an example, it often happens when a CD is released in jewel case and digipak,
with same label, date, country, barcode, etc..

Use more explicit '[no barcode]' instead of '[none]'.

A test case was added for this feature.
Merge pull request #158 from zas/releases_disambiguation

Add more info to release name in other releases list if needed (improved disambiguation).
Centralize Picard urls in PICARD_URLS dict.

Introduce `webbrowser2.goto()`, which open picard urls by id.
Merge pull request #182 from zas/url_constants

Centralize Picard urls in PICARD_URLS dict, introduce webbrowser2.goto()
Open "Install plugin" dialog with user's home directory instead of '/'
Merge pull request #185 from zas/pluginshomepath

Open "Install plugin" dialog with user's home directory instead of '/'