Use nodm rather than inittab to autostart X11 DE's

This makes switching between DE's way easier. Now a user can just deinstall the current DE's meta package (e.g. apk del postmarketos-ui-hildon) and install another one. Since nodm only works for X11, we'll still have to use something else for Wayland-based environments like Plasma Mobile or Weston.

We can tell it to run any shell command, so running something like ~/.xinitrc is possible (provided that file is available in the home directory of the user that nodm is told to log in as) using NODM_XSESSION.

Problem with this is though that everything starts as root again. Not sure what to do about that though, as /usr/sbin/nodm only allows itself to be ran as root. It can be told to login as an user using the NODM_USER environment variable, but this requires a username rather than a UID. Since this is different for everyone, we can't really hardcode that one...

This checks a few boxes for #88 (closed).

Before anyone merges this, please test it out with every X11 DE. They should all autostart like before, they did for me anyway, but I want to be sure.