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
Divide the values into sections

Breaks backward compatibility.

Introduces sections that will be later used for the configuration UI generator.

Changes comments to have the first line as short as possible, again to be used in the UI generator.

UI generator - coming some time in this decade.
Merge pull request #169 from MartinBriza/configuration

Rewrote the config to a more rigid state