Introduce restricted modes

[rsubtil]

Restricted modes limit some functionality of the app in order to hide settings or exposure to the system's OS. They should basically limit access to the settings panel.

For now there can be two modes:

• Kiosk-mode: For when RetroHub is used on a public setting. Prevents access to settings panel.
• Kid-mode: Extends kiosk-mode by also not showing games above a certain rating (#288).

These modes could be disabled by either a special combination shortcut, or forcing users to manually disable it in the configuration settings.