applications: Use "application-x-executable" when app has no icon
Before
Note, in the sidebar, that two applications (DDashboard-Gtk and gnome-crc) don't have icons. Their placement in the listbox gets weird without a fallback. For this reason, this MR uses "application-x-executable" whenever an application doesn't have an icon (aka g_app_info_get_icon (info) == NULL).