[Demobrowser] Fix command line option parsing.

Passing just command line options to Chromium prevents loading of the
initial startup url. This happens because the startup url is
overridden by the last command line argument, which happens to be a
Chromium option, thus an invalid URL.

Fix consists in iterating through the command line arguments, until
a non-option argument is found (doesn't start with a dash), and use
that as the startup URL.

Change-Id: Ibe2946b25b2e023c094a85e50d2ccbd5bfce2977
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>