minor comment/doc tweaks for #690

The description of the import command was getting a bit unwieldy. Splitting up
the paragraph helped a little, but the docs could still use some love here.
more readable changelog for 1.3.5
spell check
version bump for 1.3.6
Test mbsync plugin

Exposes bug from #693
Refactor mbsync
Query string for computed fields. Fixes #693
Check for singleton directly
Test albums and singletons simultaneously
query-based fix for #693

This is @geigerzaehler's fix from b868799, just with the typo fixed
(singletons -> singleton).

An alternate (more verbose) fix would have been:

    singletons_query.subqueries.append(
        dbcore.query.BooleanQuery('singleton', True, False)
    )

i.e., pass `False` for the `fast` parameter to indicate that this is not a
fixed-field query.
changelog entry for #693
Fixed #453 and added test cases
Add more paths to search for bash completion
changelog for #695
Test and fix unicode issues in logs. Fixes #699

The helper functions for creating test fixtures now include unicode characters.
Flake8 fixes
Add more paths to search for bash completion
changelog for #699
tests: made all tests pass on cygwin. Fixes #655
replaygain: ignoring deprecation warning for threads_init in newer gi versions. Fixes #697