sddm: 0.17.0 -> 0.18.0

Bumps to the latest stable version while fixing CVE-2018-14345 [1].

Changelog [2]:
 - Support theme supplied avatars
 - Compile against Qt 5.11
 - Fix platform detection for HighDPI
 - On close, switch VT to a running session if applicable
 - Better ConsoleKit support
 - Fix authentication when non-default hidden option ReuseSession=true is used (CVE-2018-14345)
 - Hide sessions with NoDisplay=true
 - Honor PAM's ambient supplemental groups
 - Cleanup socket destruction
 - Don't quit on SIGHUP
 - Updated translations

[1] https://nvd.nist.gov/vuln/detail/CVE-2018-14345
[2] https://github.com/sddm/sddm/releases/tag/v0.18.0