Create screen edge through the Platform

Removes a diversion between X11 and Wayland. The base class Platform
creates an instance of class Edge with plugin implementations being
able to create a different type.

The X11StandalonePlugin does that and creates a WindowBasedEdge. For
this the implementation of WindowBasedEdge is moved from screenedges
into the plugin.

Unfortunately an ifdef is needed to make the screenedge test still
work as expected. This should be improved in future, e.g. have a good
way to load the platform plugin from the tests.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1419