Database scheme: add EBU R 128 Integrated Loudness and Loudness Range columns

Nothing really ground-breaking, just add those two fields
to each table that already has `bitrate`/`samplerate`/`bitdepth` fields.

Those new fields do need to be able to represent an invalid state
which is their default state, thus they are non-`NOT NULL`.
In principle, the actual field type could be `INTEGER`
(i.e. fixed point w/ 2 fractional digits), but unless we really want to
save a few bytes, it doesn't seem worthwhile.

FIXME: i'm not sure if `device-schema` should be changed too.

data/schema/schema-18.sql

@@ -0,0 +1,37 @@
+ALTER TABLE songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE subsonic_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE subsonic_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE tidal_artists_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE tidal_artists_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE tidal_albums_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE tidal_albums_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE tidal_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE tidal_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE qobuz_artists_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE qobuz_artists_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE qobuz_albums_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE qobuz_albums_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE qobuz_songs ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE qobuz_songs ADD COLUMN ebur128_loudness_range_lu REAL;
+
+ALTER TABLE playlist_items ADD COLUMN ebur128_integrated_loudness_lufs REAL;
+
+ALTER TABLE playlist_items ADD COLUMN ebur128_loudness_range_lu REAL;
+
+UPDATE schema_version SET version=18;