Set prgname to fix icon in phosh

The icon when the app is running in phosh is missing. This is due to an issue in gtk3[1]. This has caused problems in other apps, but Purism did a workaround by using dbus names[2][3]. However, as this tweaks application does not use dbus, the workaround in phosh isn't really working. As tweaks is not a cli application, setting prgname as the applicattion id should not really be an issue.

[1] https://gitlab.gnome.org/GNOME/gtk/-/issues/2822

[2] https://source.puri.sm/Librem5/OS-issues/-/issues/224

[3] https://gitlab.gnome.org/World/Phosh/phoc/-/merge_requests/282