Make it possible to disable filter-bypass by demands-attention tasks.

Summary:
Tasks which demand attention usually bypass the filters by activity
and desktop, i.e. the Task Manager will show a window that's
demanding attention even when filtering is enabled and we're on a
different activity or desktop.

This behavior is however not suitable for the Pager, which shouldn't
show windows demanding attention on all workspace delegates. This
patch adds a prop that allows the Pager to disable the filter bypass.

Reviewers: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2802