data: Add StartupNotify to desktop file
As a GTK app it supports startup notifications just fine and this allows phosh to show a proper splash screen while megapixels is starting.
Reference: https://developer-old.gnome.org/integration-guide/stable/startup-notification.html.en
Resend of https://lists.sr.ht/~martijnbraam/public-inbox/%3CYU7xVIvaPU/E9WHc%40qwark.sigxcpu.org%3E