generated appdata doesn't contain an icon when id doesn't contain .desktop suffix

[ilya-fedin]

It seems #209 is still present, at least with some app ids. We're encountering it at https://github.com/flathub/org.telegram.desktop. We sadly can't use <launchable> tag as it's just not known at build time what the desktop id will be as both snap and flatpak rename .desktop and .metainfo.xml files according to their rules. It seems the specification doesn't define how exactly the desktop id should be discovered, could it ignore the <id> tag and use the app_name?

Like

		if (launchable != NULL) {
			desktop_basename = g_string_new (as_launchable_get_value (launchable));
		} else {
			desktop_basename = g_strconcat (app_name, ".desktop", NULL);
		}

It would also be ok if the g_str_has_suffix check is just removed I guess, I just don't see how to make both flatpak and snap happy otherwise...