Update Krusader to 2.7.0.

This version is based on KDE Frameworks 5. Release notes:
https://krusader.org/release/2.7.0/release_notes.txt

While here, update pkg-message by pointing to Krusader's documentation, as
providing a list of all ports Krusader might use as external dependencies is
too much.

Reviewed by: tcberner
Differential Revision: https://reviews.freebsd.org/D15436