This PR adds FPS-style controls to the camera in addition to the existing DCC-style pan/orbit/dolly controls. Additionally, some architectural changes have been made to facilitate this and pave the way toward making all of the app controls configurable in the future. Namely, a new `input` module has been added, loosely inspired by Unreal Engine's "Enhanced Input" system (and as I understand it, conceptually similar to Unity's "new" input system as well).
The basic idea is that raw device inputs (coming from `bevy::input`) are mapped to named, app-specific "input actions" before being consumed by other parts of the application. For example, in the camera motion system, instead of checking for mouse motion while the middle mouse button is pressed, we now check for a `CameraMotion::Pan` input action event.
Input actions carry information about their lifecycle phase (`Start`, `Ongoing`, and `Stop` — a stand-in for Bevy's `just_pressed`, `pressed`, and `just_released` methods) and some type of magnitude (`f32`, `Vec2`, or `Vec3` depending on the action).
At present, we've simply moved the hard-coded device input checks from the `orbit_cam` module to the new `input` module, but theoretically, this could eventually be refactored to read the mappings between device inputs and input actions from some kind of user configuration.
Danny McGee