Improve standalone code

This finishes the needed bits for standalone to work as a whole. Adds --standalone arg to command line parse, which gets forwarded to the necessary classes to handle paths in a proper way.

Also QSettings are abstracted in a LeapSettings class to improve this part and also handle these settings in a standalone way when needed.

This also adds a way to pass platform dependant process environments to the vpn process, which is particularly used for setting LD_LIBRARY_PATH so we can be sure vpn loads our bundled dynamic libs when present.