Make controls configurable

[jorisvddonk]

At the moment, controls use Godot's native Input handling systems, which abstract keyboard/mouse/controllers. This makes things technically configurable.

There is, however, no UI for configuring the controls. It'd be great to add this!

• Add UI for making controls configurable.
• Ensure control bindings are persisted to a configuration file somehow. (via a Resource?)
• Ensure help UI (f1) shows the configured controls instead of the hardcoded text it has now
• Ensure hud rim uses the correct labels for the Stardrifter's main screen keyboard shortcuts