eglfs refactor: move window creation into qeglfswindow

eglfs refactor: move window creation into qeglfswindow

This potentially allows the creation of multiple QWindows. The platform
context is now in a seperate file and the integration provides a new
instance of the context allowing creation of multiple contexts.

Change-Id: If2b6fa29b573d87c0a4cd0a8eff1f044bd1ff9b8
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>