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