Remove seperate role repesentation

The role and instrument tables have been merged into one (the instrument
table). There are not that many roles that aren't instruments and it is
much simpler to mange this way. The role editor and role selector have
been removed and the instrument related UI parts have been modified
accordingly.

lib/database.moor

@@ -4,6 +4,8 @@ CREATE TABLE persons (
     last_name TEXT NOT NULL
 );
 
+-- This represents real instruments as well as other roles that can be played
+-- in a recording.
 CREATE TABLE instruments (
     id INTEGER NOT NULL PRIMARY KEY,
     name TEXT NOT NULL
@@ -28,11 +30,6 @@ CREATE TABLE ensembles (
     name TEXT NOT NULL
 );
 
-CREATE TABLE roles (
-    id INTEGER NOT NULL PRIMARY KEY,
-    name TEXT NOT NULL
-);
-
 CREATE TABLE recordings (
     id INTEGER NOT NULL PRIMARY KEY,
     work INTEGER REFERENCES works(id)
@@ -42,7 +39,7 @@ CREATE TABLE performances (
     recording INTEGER NOT NULL REFERENCES recordings(id) ON DELETE CASCADE,
     person INTEGER REFERENCES persons(id) ON DELETE CASCADE,
     ensemble INTEGER REFERENCES ensembles(id) ON DELETE CASCADE,
-    role INTEGER REFERENCES roles(id)
+    role INTEGER REFERENCES instruments(id)
 );
 
 allPersons:
@@ -84,12 +81,6 @@ SELECT * FROM ensembles ORDER BY name;
 ensembleById:
 SELECT * FROM ensembles WHERE id = :id LIMIT 1;
 
-allRoles:
-SELECT * FROM roles ORDER BY name;
-
-roleById:
-SELECT * FROM roles WHERE id = :id LIMIT 1;
-
 recordingById:
 SELECT * FROM recordings WHERE id = :id;