Do not use gloabal vars for the command line parsing

Also factor out an open_files helper, this will be used to also
implement application::open in the next commit
Handle also application::open

Even if we do our own command line parsing, we still need to implement
open since the dbus method is used when activating files from nautilus.

https://bugzilla.gnome.org/show_bug.cgi?id=734175