Added handling of QT_QUICK_CONTROLS_STYLE environment variable.

Now one can choose which set of styles should be used by the controls.
The value should be the name of the QtQuick/Controls/Styles subdir
(eg. QT_QUICK_CONTROLS_STYLE=Desktop).

NOTE: The environment variable is for internal use and may be removed
in the future.

Change-Id: I23919270cd1e2406534fafe1134739d145a93538
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>