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.

src/core/database.cpp

@@ -48,7 +48,7 @@
 #include "scopedtransaction.h"
 
 const char *Database::kDatabaseFilename = "strawberry.db";
-const int Database::kSchemaVersion = 17;
+const int Database::kSchemaVersion = 18;
 const int Database::kMinSupportedSchemaVersion = 10;
 const char *Database::kMagicAllSongsTables = "%allsongstables";