Add support for new touch screen edge actions to declarative KWin scripts

Summary:
This change introduces a new property mode in ScreenEdgeItem to switch
between Pointer and Touch mode. The touch mode interacts with the new
touch screen edge gestures through a QAction.

Test Plan: New test case added

Reviewers: KWin, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5272