Set StartupWMClass to signal

When running under Wayland, Signal sets an app id of signal. This can be seen by setting WAYLAND_DEBUG=1:

[2279573.109]  -> xdg_surface@39.get_toplevel(new id xdg_toplevel@40)
[2279573.114]  -> xdg_toplevel@40.set_app_id("signal")
[2279573.118]  -> xdg_toplevel@40.set_title("Signal")

Without this change, the Signal window is not recognised as belonging to the signal-desktop.desktop application, and the generic app icon is used:

This change does not appear to affect anything when Signal runs under X11.

See https://github.com/signalapp/Signal-Desktop/issues/6868 upstream for more context.