Toggle session manager button availability depending on session selection

In the session manager window rename, switch to and delete buttons
were always enabled, despite having no actions without a selected
session.

These buttons are now enabled depending on selection of a session.

Co-Developed-By: Maximilian Schier <maximilian.schier@stud.uni-hannover.de>
Co-Developed-By: Niclas Wüstenbecker <niclas.wuestenbecker@stud.uni-hannover.de>

CCBUG:422486