[Pager] Don't show a window list on hover if there is only one window

Summary: If there is only one visible window on the virtual desktop show the title directly instead of "1 Window" + one-entry list.

Test Plan:
Before:

After:

Reviewers: hein, broulik, ngraham

Reviewed By: hein, ngraham

Subscribers: mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14026