[Digital Clock] Add tooltip for agenda items with too long description

Also, when an appointment has no description, let the title use both lines of text,
and center them vertically if it doesn't need both lines.
The tooltip is only enabled if the title is actually elided, otherwise it could be bothersome to have.

BUG: 378970
FIXED-IN: 5.12.0

Differential Revision: https://phabricator.kde.org/D9641