Don't wait for the greeter before running the display setup script

Currently, the display setup script is ran at two different places:
- With auto login, it is ran after X is ready
- Without auto login, it is ran after the greeter is ready

The script is now always ran after X is ready, this seem to work
around Qt bug #39552
Fix greeter socket error with Qt 5 due to the socket needing root access

The socket is created from the daemon which is ran as root, and it is
given UserAccessOption which means only the root user can access it.

Considering the greeter is ran under the sddm user, this prevents it
from connecting to the daemon in some cases.
Merge branch 'AnAkIn1-displaysetupscriptfix'
Change socket owner and group

Keep the socket open with user access but change owner and group to
sddm to avoid permission denied errors from the greeter.

This is not done in test mode as everything runs with one user.
Merge pull request #214 from plfiorini/socket-permission

Change socket owner and group
Remove custom session

The "Custom" session runs ~/.xsession, if the script is not available
sddm restarts the greeter and display server because execution failed.

We should force users to write proper desktop entries instead of executing
unknown commands.

Closes #220