Summary:
Source code reorganization:
The base class AbstractBackend got renamed to Platform, thus the
"backends" are "platforms" now. As they are plugins they should go
together with other KWin plugins which are nowadays in the folder
plugins.

So new location is plugins/platforms/

Reviewers: #plasma, sebas

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1353