Add --hide-icon to start Solaar with a hidden icon

The icon is still created, but not displayed. Addresses a request in
https://github.com/pwr/Solaar/issues/84

add_main_option_entries[1] could be used, but is only supported in
GLib 2.40 and newer which excludes Ubuntu 13.10 Saucy and Debian Wheezy
and earlier.

 [1]: https://wiki.gnome.org/HowDoI/GtkApplication/CommandLine