Redo how works are stored

Before works where stored using a complicated tree structure where each
part references its parent. Now it's a flat structure that keeps track
of the relations using a new field named part_level. The corresponding
query was simplified accordingly. Also, the updateWork function was
rewritten.

lib/database.moor

@@ -14,7 +14,8 @@ CREATE TABLE works (
     composer INTEGER REFERENCES persons(id),
     title TEXT NOT NULL,
     part_of INTEGER REFERENCES works(id) ON DELETE CASCADE,
-    part_index INTEGER
+    part_index INTEGER,
+    part_level INTEGER
 );
 
 CREATE TABLE instrumentations (
@@ -37,16 +38,13 @@ SELECT * FROM instruments WHERE id = :id LIMIT 1;
 workById:
 SELECT * FROM works WHERE id = :id LIMIT 1;
 
-worksByComposer:
-WITH RECURSIVE work_parts(id, part_of) AS (
-    SELECT id, part_of FROM works WHERE composer = :composerId
-    UNION ALL
-    SELECT works.id, works.part_of FROM works
-        JOIN work_parts ON works.id = work_parts.part_of
-)
-SELECT works.* FROM works
-    JOIN work_parts ON works.id = work_parts.id
-    WHERE work_parts.part_of IS NULL;
+workParts:
+SELECT * FROM works WHERE part_of = :id ORDER BY part_index;
+
+-- TODO: Uncomment this once https://github.com/simolus3/moor/issues/453 is fixed.
+-- worksByComposer(:id AS INTEGER):
+-- SELECT DISTINCT A.* FROM works A, works B ON A.id = B.part_of
+--     WHERE A.composer = :id OR B.composer = :id;
 
 instrumentsByWork:
 SELECT instruments.* FROM instrumentations