Focus session from notification

Adds a "Show session" button to notifications, which, when clicked, will
focus the originating session.

Original patch by Kasper Laudrup at bug 305162, then updated by Martin
T. H. Sandsmark.

Now updated and extended to silence/activity/process termination
notifications, adding support for focusing a TerminalDisplay inside a
hierarchy of ViewSplitters inside a TabbedViewContainer, and using
KWindowSystem::forceActiveWindow() instead of ::activateWindow().

BUG: 305162
BUG: 344208