Add OS Update notification and Installer support

[pastaq]

Immutable OS's such as SteamOS and ChimeraOS provide OS updates via a single system image. Steam in gamepadui mode has a notification and system updater button that hooks into the system updater program to perform version upgrades of the underlying OS. We can hook into these daemons to provide the same functionality in opengamepadui using the Platform global. The feature should:

• Detect when a system update is available on a supported OS.
• Notify the user of an available update, if desired.
• Install an update using the systems native system updater and provide a notification when installed.
• Allow the user to set automatic updates, if desired.

No additional permissions should need to be created for this as it should assume the required polkits exist in the OS. If they don't , then the feature is not supported.

[pastaq]

@ShadowApex what OS's have suport currently?