User settings

The new bevy_settings crate provides a framework for user settings and persistent preferences. This can include things like:

• Music and sound volume controls
• Graphics options
• Window position and size
• Whether or not to show the game tutorial
• "Don't show this dialog again"

In general, a user preference is any persistent property that is set by user action (either explicitly or implicitly), and whose lifetime isn't limited to a single saved game file.

Preferences are defined using bevy_reflect annotations, and are automatically inserted as resources when the settings framework starts up.

See the examples/app/persisting_preferences for a simple example of how to use the framework.