core: provide infrastructure to store last selected course per profile

Attention: with this commit the profiles table got a new column.
Because this version of ktouch is still unreleased no database migration
step is provided. Either delete your profiles database
(~/.kde4/share/apps/ktouch/profiles.db) or adapt the table manually.

You may use the following SQL command:

ALTER TABLE profiles ADD COLUMN last_used_course_id TEXT;