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'
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
Rewrote the config to a more rigid state

Resolves sddm/sddm#39

More configuration instances now available. Every instance is a separate class.

Sections are supported.

Serialization handled in a more sensitive way - does not remove comments.

Type handling is unified, using the QTextStream shift operators.

There are also some basic tests, to be further improved...

Probably the most overengineered configuration parser you've ever witnessed.
Move LastUser and LastSession into /var/lib/sddm/state.conf