Prevent cover images from skipping numbers

This is a bug introduced with fc7470eee076c06d925598d3f29e7eca7cb089c6:
When saving multiple images with the generic filename, only odd numbers
would be appended to the filename, because the "while exists(foo)" check
is always true for every image except the first one at least once,
causing "i" to be incremented twice for every image (except for the
first one).
save_images: Use separate counters for different base filenames
only autoanalyze files that can be analyzed
Add filename related parameters for file naming scripts
Merge branch 'autoanalyze' of https://github.com/mineo/picard into mineo-autoanalyze
Fix initial shift-click in file browser (PICARD-314)
Merge branch 'picard-278'
HACKING.txt: Run pybabel on contrib/ as well
Update picard.pot with strings from contrib/
Fix various tagdiff oddities (PICARD-256, PICARD-255)
Remove duplicate counter var
Correctly edit different multi-valued tags
Update translations from Transifex (tx pull)
Merge branch 'master' of github.com:musicbrainz/picard
Update NEWS.txt
Increase the version number
Try to solve UI freezes

Calling setCurrentIndex from currentChanged causes an infinite loop, breaking
the UI. But without that line, there are other very strange effects: see
PICARD-248. The only thing that currentChanged was added for was to make sure
the next item is selected after something is removed, so that keyboard
navigation is easier. This implements a new workaround for that without using
currentChanged.
Add the release date for 1.1
Options crashes when naming script produces empty filename
Merge branch 'master' of github.com:musicbrainz/picard