Bug#769465: improved batch processing workflow

[Daniel Pocock]

Package: flactag
Version: 2.0.4-3
Severity: wishlist

Currently, when using the batch mode to operate on many flac files,
there is a pause between each file while flactag queries MusicBrainz.
The pause can last 5-10 seconds depending upon network conditions and
the MusicBrainz service availability.

E.g. if the user specifies ten files, then for each file:
1. 5 second pause
2. flactag shows possible match(es)
3. user chooses how to proceed
4. goto step 1 (user waiting 5-10 seconds before interacting with
flactag again)

This pause between every operation can be frustrating for the user.

A better strategy would be:
- scan all the flac files mentioned on the command line
- obtain all possible data for all of the files from MusicBrainz (maybe
with parallel requests from a thread pool for better performance), show
a progress bar while this is happening
- show a GUI listing each title and allowing the user to quickly move
between them in a list or using forward/back buttons

Once the user reaches the GUI stage, there should not be any pauses as
all relevant data has already been fetched from MusicBrainz.