Allow disabling filtering by SkipTaskbar.

Summary:
This makes TaskFilterProxyModel useful for filtering a source tasks
model (e.g. WindowTasksModel) without implicitly filtering out tasks
that don't want to be shown on a 'task bar'.

Reviewers: Plasma, mart, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2380