Simplify and improve music indexers management

Summary:
start baloo first and if it is not available, start local file indexer

remove management of track discovery sources

simplify management of tracks by removing management of which source has
discovered a track

should allow users to easily switch from Baloo to local file system as
tracks source

improve management of tracks sources such that only baloo or local file
system scanning is enabled

no longer emit a signal when Elisa cannot watch a file for changes

refactor the way we check for Baloo availability

filter tracks by the configured paths when getting all tracks from Baloo

use the same list of paths than plain file system scanning

BUG: 393106

remove double checking for baloo configuration: not needed

Baloo configuration is now checked before going into
LocalBalooFileListing: no need to do it a second time

add support for qt logging category and use it for file indexers

improve management of indexers state and user reporting

convert Loc...