Rework the HTML5 shell to avoid hacks:

• launcher, homescreen and background should be launched from systemd.
• A way to properly order the ready events should be done, so all applications get the right side.
• This should be done after fixing the concurrency problem described in SPEC-4252 -> It should be possible to launch the 3 applications in parallel.

Discussion about this happened at https://gerrit.automotivelinux.org/gerrit/c/apps/html5-homescreen/+/27323