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