Selecting one entity at a time...
* Affects both 2d and 3d modes seemingly equally
* We seem to not be de-selecting something...
* Does it happen when you play the game fast/slow?
    * Yes
* It only seems to happen when we capture a piece.

New architecture:
* Instead of inserting a "Selected" marker, emit a "Selected" event.
    * Contextually this acts differently based on:
        * If no entities are selected, insert the selected marker on that entity.
        * If an entity is already marked selected, we emit a move event and de-select all entities.
* Q: Should we "Select Event" a BoardIndex?
    * Pros:
        * Display agnostic.
        * Easy to keep in sync between displays.
    * Cons:
        * Possibly umm... actually I can't think of any.
* Emit "Selected" event which includes the BoardIndex selected.
    * If a piece exists at that index:
        * If a piece is already selected, emit a "Move" event and de-select the selected piece.
        * Else mark it as selected in 2d and 3d.