Remove the hack modifying album.metadata['album'] for display purpose

This hack was leading to cluttered debug messages since __repr__() was returning
status message in place of album's title.

Instead use a `status` property which is displayed in place of the title when it isn't None.
Fix debug message "Add files <generator object <genexpr> at ..."

Not very useful, replaced by a more useful debug message (later in code, since exclusion of files is already logged)
Fix up "Waiting 0 ms before ..." debug message

- last_ms has sub-milliseconds precision
- request_delay - last_ms can be < 1.0 (and it is a float)
- message use %d to convert value from float to int (so 0.5 is displayed 0 ms)
- QTimer.start() only supports milliseconds precision as int (so 0.5 will prolly be converted to 0)
- if delay is required, better wait more than not enough, so, ie.,  0.7 ms delay is converted to 1 ms
- when delay is required, 0 delay doesn't make sense, first branch of the if condition handled that
- int(math.ceil(delay)) does the job
- int() cast is needed with python 2 (it returns a float), python 3 math.ceil() returns int
Fix issues in Tags from File Names...

1. Replace all \ with / in path not just last one so it works properly
on Windows.
2. Set combo index to show last used string
3. Strip leading zeros from numeric tags like tracknumber.
Merge pull request #320 from zas/album.metadata.hack.fix

Remove the hack modifying album.metadata['album'] for display purpose
Merge pull request #324 from zas/add_files_debug_fix

Fix debug message "Add files <generator object <genexpr> at ..."
Merge pull request #325 from zas/request_delay_fix

Fix up "Waiting 0 ms before ..." debug message
Merge pull request #327 from Sophist-UK/sophist_fix_tagstofilenames

Fix issues in Tags from File Names...
Strip tracknumber from title if leading
Improve clustering by grouping by directory
Tweak to not overwrite artist if it exists
Remove debug print.
Simplify code for readability.
Move album_artist_from_path to utils

and add test.
Fix typo
Merge pull request #328 from Sophist-UK/sophist_improve_clustering

Improve clustering using directory structure if no existing album tags
Add missing whitelines + album_artist_from_path() description