Add support for display setup script

It is done the same way as KDM does it.

With autologin, it will be ran right after the display server starts.
Without autologin, it will be ran right after the greeter has
connected to the daemon. This is needed, otherwise it'll just
show a black screen on an Optimus laptop.
Detect if X is ready with the SIGUSR1 signal

Don't retry the connection, rather detect when X is ready with SIGUSR1.

From Xserver man page:

  "When the server starts, it checks to see if it has inherited SIGUSR1
   as SIG_IGN instead of the usual SIG_DFL. In this case, the server
   sends a SIGUSR1 to its parent process after it has set up the various
   connection schemes. Xdm uses this feature to recognize when
   connecting to the server is possible."
Merge branch 'AnAkIn1-displaysetupscript'
Start sddm.service after plymouth-quit.service

This will make sddm.service run after Plymouth, but upstream need to
be patch for retaining the splash.

See issue #196.
Don't reset background when starting the X server

See issue #196.
Merge pull request #198 from plfiorini/plymouth

Initial Plymouth support
Rename everything related to QAuth to something along the lines of SDDM::Auth
Fix typo
Merge pull request #201 from MartinBriza/qauth_rename

Rename everything related to QAuth to something along the lines of SDDM::Auth
Drop non-free font and use Oxygen

It was pointed out that Geosans Light is not a free font and that
causes licensing issues for distributions such as Fedora.

Remove the font and replace with Oxygen which would probably be
already installed by our users.

Closes #109
Merge pull request #204 from plfiorini/oxygenfont

Drop non-free font and use Oxygen
Initialize sigactions in signal handlers
Merge pull request #210 from MartinBriza/signals_initialize

Initialize sigactions in signal handlers
Remove unnecessary Session class from the daemon
Silence CMake 3.0 warnings due to policy changes

Closes #206
Set policy CMP0043 only if it exists

Guard the command to avoid errors with CMake versions older than 3.0.
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 pull request #213 from plfiorini/cmake3

Set policy CMP0043 only if it exists
Merge branch 'AnAkIn1-displaysetupscriptfix'