Move requiresCompositing from Application to Platform

Summary:
This change reduces the windowing system specific code pathes. Instead
of checking whether we are on X11 or Wayland to decide whether
compositing is required, we just ask the Platform.

The default is true, only x11 standalone allows to not require
compositing.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1575