Handle ambiguous column names in queries involving 'any' field and a relation field

[snejus]

Description

Fixes a sqlite3.OperationalError that occurs when querying any field and a field that only exists in the relation table. For exaple, trying to list albums that contain keyword and contain a track with foo in its title:

beet list -a keyword title:foo

Root Cause

SQLite fails when JOINs contain ambiguous column references. This happened because:

• any Album field search looks at album, albumartist and genre fields.
• The second part of the query title:foo queries a field in the items table, which got
  JOINed with albums
• Some fields (like album) exist in both items and albums tables, thus SQLite couldn't resolve which table's column to use

Changes

• Centralize query construction in LibModel with consistent table qualification
• Add methods:
  • field_query() - Creates table-qualified field queries
  • any_field_query() - Creates multi-field OR queries
  • any_writable_media_field_query() - Similar to the above but for BPD / media files
  • match_all_query() - Creates multi-field AND queries
• Remove AnyFieldQuery in favor of composed OrQuery
• Add tests for shared field querying

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)