Further clean up addons CMake scripts.

Summary:
- Reduce addon dependencies to the bare minimum required.
- Move source files out of sessionapplet/engine directory for
  consistency with the other addons.
- Ensure each addon is built only if all of its dependencies are found.
- Remove .kateconfig file in close-expect-like addon directory.
- Remove comments with no added value.

Test Plan: Make sure builds as before with the same (or an extended) set of addons.

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D22773