Database: Add *sort fields, bpm, mood, initial_key

Upgrade from schema version 20 to 21. This includes:

- six fields for sort tags
- new fields bpm, mood, initial_key

See https://github.com/strawberrymusicplayer/strawberry/pull/1779#pullrequestreview-3003042802

src/core/database.cpp

@@ -50,7 +50,7 @@
 
 using namespace Qt::Literals::StringLiterals;
 
-const int Database::kSchemaVersion = 20;
+const int Database::kSchemaVersion = 21;
 
 namespace {
 constexpr char kDatabaseFilename[] = "strawberry.db";