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/data.qrc

@@ -9,6 +9,7 @@
         <file>schema/schema-15.sql</file>
         <file>schema/schema-16.sql</file>
         <file>schema/schema-17.sql</file>
+        <file>schema/schema-18.sql</file>
         <file>schema/device-schema.sql</file>
         <file>style/strawberry.css</file>
         <file>style/smartplaylistsearchterm.css</file>