- made public headers compilable individually
- easier access to settings section delegates
- removed module singleton accessor
- moved non-trivial definitions into cpp files
- code & documentation cleanup
#UpgradeNotes:
- instead of ISettingsModule::Get() use FModuleManager::GetModulePtr<ISettingsModule>("Settings")
- instead of using FSettingsSectionDelegates assign delegates directly through the new ISettingsSection methods
[CL 2340711 by Max Preussner in Main branch]
