Made camera controls customizable

[Cons-Cat]

I use the Hands Down family instead of the QWERTY keyboard layout, and I use home-row mod keys. For these reasons, I (and anybody else with this fairly popular kind of setup, at least among keyboard nerds) need the inputs to be rebindable to one or multiple keys. In my current project, I now have this line:

  app.camera.set_movement_keys({lava::key::m},
                               {lava::key::n, lava::key::left_control},
                               {lava::key::s, lava::key::left_super},
                               {lava::key::t, lava::key::left_alt});

This PR keeps WASD as the default control scheme.

[Cons-Cat]

With this system, arrow keys and WASD could both move the camera by default. Do you think that's a good idea I should add?

[TheLavaBlock]

Looks good, I'll adjust a few things later, but for now we'll do the PR 👍