[WPE][GTK] webkit-flatpak intercepts --help for other commands

[WPE][GTK] webkit-flatpak intercepts --help for other commands
https://bugs.webkit.org/show_bug.cgi?id=189058

The approach of run_in_sandbox_if_available is that we basically have the
same set of arguments to setup the sandbox (ie. port name, build type, etc...)
and we can pass those args first to setup the flatpak launcher object
and then to the underlying script. That doesn't work well with the `--help` argument
as once processed it 1. prints the help (which make no sense to the user of the calling script)
2. exits the app - The solution is to just make sure that `--help` is not used when using
flatpakutils from any script that is not `webkit-flatpak` itself.

Patch by Thibault Saunier <tsaunier@igalia.com> on 2018-09-20
Reviewed by Michael Catanzaro.

* flatpak/flatpakutils.py:
(WebkitFlatpak.load_from_args):
(run_in_sandbox_if_available):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@236261 268f45cc-cd09-0410-ab3c-d52691b4dbfc