Add a pure virtual method to Platform to describe the supported compositors

Summary:
So far KWin does not know which Compositors the platform actually
supports. This results in KWin happily trying to use the OpenGL
compositor on fbdev or the QPainter compositor on hwcomposer although
that is obviously going to fail as the platform doesn't support this.

By adding a pure virtual method all Platforms can define what they
support. In a later step the Compositor can use this to create an
appropriate scene and also perform proper fallback handling in case the
scene creation fails.

Test Plan: Compiles

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8316