Allow custom cursors.

[StarArawn]

It should be fairly easy to allow for custom cursors instead of using the default OS's cursors.

[Braymatter]

I haven't found something in Bevy that lets you load custom cursors. I would disable the bevy cursor visibility and render a sprite at that cursor's position every frame.

We could add a Resource that acts associates a CursorIcon with a Sprite/Bundle and have a new system handle visibility/updating the sprite.