Provide better activity stats in the status bar

Partial solution to PICARD-478
Remove un-needed num_saves method
Provide more consistent status bar info and code

Still need to make "Unmatched Files" and "Clusters" FileTreeItems
consistent.
Rename stuff to better reflect usage
Add cluster count
Address review comments
Merge branch 'sophist_improve_status_counters' of git://github.com/Sophist-UK/picard into Sophist-UK-sophist_improve_status_counters
Provide better activity stats in the status bar

Partial solution to PICARD-478
Remove un-needed num_saves method
Provide more consistent status bar info and code

Still need to make "Unmatched Files" and "Clusters" FileTreeItems
consistent.
Rename stuff to better reflect usage
Add cluster count
Address review comments
On Windows display drives in sorted order in FileBrowser
Do not add cover art relation url twice (ie. cdbaby)

Stop loop if first url is added because second one will be removed later anyway.
It happens with cdbaby urls (ie. http://musicbrainz.org/release/6e228dfa-b0c7-4987-a36d-7ac14541ae66)


Debug output without this patch:
D: 139947473626880 11:15:32 There are no suitable images in the cover art archive for 6e228dfa-b0c7-4987-a36d-7ac14541ae66
D: 139947473626880 11:15:32 Adding front image PyQt4.QtCore.QUrl(u'http://cdbaby.name/p/r/prozak.jpg')
D: 139947473626880 11:15:32 Adding front image PyQt4.QtCore.QUrl(u'http://www.cdbaby.com/cd/prozak')
D: 139947473626880 11:15:32 'Downloading http://cdbaby.name:80/p/r/prozak.jpg'


Debug output with this patch:
D: 140074644940544 11:16:55 There are no suitable images in the cover art archive for 6e228dfa-b0c7-4987-a36d-7ac14541ae66
D: 140074644940544 11:16:55 Adding front image PyQt4.QtCore.QUrl(u'http://cdbaby.name/p/r/prozak.jpg')
D: 140074644940544 11:16:55 'Downloading http://cdbaby.name:80/p/r/prozak.jpg'
Merge branch 'sophist_improve_status_counters' of git://github.com/Sophist-UK/picard into Sophist-UK-sophist_improve_status_counters
Merge branch 'Sophist-UK-sophist_improve_status_counters'
small whitespace fix
Merge branch 'zas-cdbaby'