Initial import of tasks application

This initial import is a heavily modified version of the code I had
here, as Arse was modified for other purposes in the meantime and the
application no longer worked with it.

In addition:
 * I did not import the user management part yet,
 * task dependencies are supported in-base, but there is no interface
for that yet.

database.sql

@@ -0,0 +1,17 @@
+\i database/config.sql
+\c :db_name
+
+BEGIN;
+
+-- Tables from the main database structure
+\i database/create-tables.sql
+
+-- Items tree management and associated functions
+\i database/items-tree-triggers.sql
+\i database/items-functions.sql
+
+-- Task management and task dependencies
+\i database/tasks-functions.sql
+\i database/task-dependencies.sql
+
+COMMIT;