Document units of MetaData::length()
Get rid of constants.h, use QStandardPaths instead
Fix formattedLength msecs to string conversion
Print debug info with number of loaded plugins
Make Playlist::loadMetaDataRunnable less blocking

As the entire point of the runnable is to load metadata asynchronously,
it makes no sense to hold a mutex the entire time the runnable runs. We
now instead lock only to modify the metadata and then emit dataChanged().

Since the Playlist model emits dataChanged() for only one column, we need
to map the signal in PlaylistProxyModel to apply to all columns.
UTF-8 support in M3U helper
Fix crash when playlist is cleared while loadMetaDataRunnable is still running
Replace the custom settings icons by XDG ones
Remove the old TepSonic icon and rename the currently used ones
Drop support for MySQL

Supporting multiple database engines is a lot of work, so let's
send MySQL to sillicon heaven and only support SQLite. We can
bring MySQL backend back later.