Add GLContextHelper class for EGL context creation.

Shared EGL contexts should always be created on the same
thread, as otherwise context creation might fail.
We use a GLContextHelper singleton that is initialized
on startup and creates the contexts when requested
through a BlockingQueuedConnection.

There is a pretty and a correct solution for this problem.
This is the pretty one. It is based on the assumption
that Chromium decides to use the same or a similar enough
configuration for the EGLContext as Qt previously did.
But we prefer pretty over correct as the correct solution
would potentially impose a layering violation.

This is enabled for all embedded linux devices.

Change-Id: I910cc90d0f87fd6d1fe0a475b17ba56cd8c503f6
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>