Do screen filtering by screen geometry rather than number.

Summary:
Avoids problems stemming from libtaskmanager and plasma-framework's
AppletInterface::screen numbering screens differently (QScreen order
vs. who knows what).

CCBUG:365246

Reviewers: #plasma, graesslin, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2143