This PR implements basic object selection, with help from `bevy_mod_picking` and
`bevy_mod_outline`.
### Work Done
* Click to select objects in the world
* Selected objects become visually highlighted
* The selected entity is available globally on the `Res<Selection>` resource
* Selected entities can also be queried via the `Selected` marker component
* Pressing `F` will "focus" the camera on the selected entity
* Pressing `Esc` will clear the active selection
### Known Issues
* We're currently using `bevy_mod_picking`'s "Selection" feature, which isn't
aware of our own input events. As a consequence, it will select the object
under the cursor when releasing `LMB` after orbiting the camera, which is not
what we want. This should be pretty easily fixable by dropping
`bevy_mod_picking`'s selection feature and reading the object under the
cursor in our own click listener that only runs if the LMB event is
unhandled.
* The skinned mesh is not currently selectable. This is likely fixable by
adjusting the logic in `scene::mark_scene_objects`... somehow. The `Gltf`
data structure is kind of a nightmare to work with, so this will take some
investigation.
### Future Enhancements
* For parity with other DCC apps, the "Focus" action should also adjust the
camera zoom level to "frame" the selected object in the view. Punting on that
for now because I'm not super confident about the math involved.
* Group selection with Shift/Ctrl and/or marquee selection would be really nice
to have, but probably not critical for now.
Danny McGee