main/postmarketos-ui-phosh: firefox improvements

Display the app icon properly, instead of the "unknown" icon, in the activity view. Phosh attempts to open a .desktop file with the name of the gtk app-id, but the app-id is "firefox" and the file is named org.mozilla.firefox.desktop (see related commit, where it was renamed for a good reason). The way Phosh is doing the name lookup doesn't always work for other apps too, phosh_get_desktop_app_info_for_app_id() has workarounds e.g. for gnome-control-center hardcoded. As workaround, add a .desktop file that makes the icon show up properly, but is not displayed anywhere else (NoDisplay=true).

Replace epiphany with FF in the default favorite applications in phosh (top 4 displayed at the top). We already install FF instead of epiphany and so the 4th place is currently empty.

Related: https://gitlab.alpinelinux.org/alpine/aports/-/commit/2d291a4afc2c71e25c8fd9fff188eff436b80580