Make display managers consistent

Right now we use tinydm for some UIs, lightdm for others.

1. tinydm just does an autologin and makes sense in the following situation:

• Your device is encrypted with FDE, so you already need to type in a password on boot. This is the most sensible use case for phones.
• You want it to automatically login without password (e.g. when you don't store personal data on the device and just want to use it to launch something like kodi to watch movies, or open a browser and display some data on a monitor)

2. A full DM like lightdm (or phog or ...) makes sense if:

• you don't use FDE and want to type in a password on boot
• you want to be able to switch between multiple UIs
• you want to be able to switch between multiple users

Let's come up with a consistent story here. How about:

• we use tinydm by default everywhere
• but provide a sane way to change to a full DM? Like installing one package or running one command that sets everything up. (automatize this)
  • let's standardize on what the full DM would be?

EDIT: Bart had better idea for defaults, autologin for mobile and no autologin for desktop, see comment below.

This issue is for discussing this further and coming up with something consistent. In the meantime I'll revert the tinydm -> lightdm change we just did for mate and xfce4, because it broke the second use case.