Fix override cursor issue with QtQuick2ApplicationViewer.

If the override cursor was set before
QtQuick2ApplicationViewer was shown then the cursor
was the default cursor and not the override cursor.
The new cursor was applied only if the window was
visible and the cursor has been set. Now the cursor
is applied also if override cursor is set.

Change-Id: I88618c719e43b2802e209bef440461c532d19f48
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Only query for the theme if it is available in the GTKStyle

If the theme is not available then it should not try to use it when
getting the file system icon as it will cause a number of messages to be
outputted to the console as it is an invalid call.

Change-Id: I33003568f6785ee423c4a6e079a6c62c95145cbc
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>