Move default category into database (#7676)

app/src/main/sqldelight/migrations/19.sqm

@@ -0,0 +1,10 @@
+-- Insert Default category
+INSERT OR IGNORE INTO categories(_id, name, sort, flags) VALUES (0, "", -1, 0);
+-- Disallow deletion of default category
+CREATE TRIGGER IF NOT EXISTS system_category_delete_trigger BEFORE DELETE
+ON categories
+BEGIN SELECT CASE
+    WHEN old._id <= 0 THEN
+        RAISE(ABORT, "System category can't be deleted")
+    END;
+END;