Revamp access to FAF settings file

FAF uses QSettings to save and load application-wide settings. It has a Settings singleton defined in the config module that abstracts the saving and loading of settings, as well as setting settings without saving them at exit.

The way FAF actually uses them is messy. Some code uses Settings with hardcoded config names and types, some code creates magic "persisted properties", some code steals the actual QSettings and uses it directly. This can be cleaned up to use constants and avoid using the QSettings object.