Create a blaster-controls custom element

[pookage]

• [ ] Define a new <blaster-controls> custom-element
• [ ] The element should render in its template two buttons
• [ ] The buttons should be placed either side of the screen in the vertical center
• [ ] The buttons should be circular, semi-transparent, and have :active, :hover: :focus styles etc
  • NOTE: remember to make use of that fancy new focus technique that I've forgotten the name of
• [ ] The element should accept left and right attributes, which are selectors for the respective <a-blaster> entities (#6) in the <a-scene>
• [ ] Tapping the left button should emit a "fire" event on the entity selected by the left attribute
• [ ] Tapping the right button should emit a "fire" event on the entity selected by the right attribute
• [ ] The element should listen for the mouseup event
• [ ] If the left mouse-button is clicked, the "fire" event should be emitted as if the left-button had been pressed
• [ ] If the right mouse-button is clicked, the "fire" event should be emitted as if the right-button had been pressed

[pookage]

I'm not sure whether the a-frame scene will be ready by the time the left and right attributes grab the entities; so we should do some kind of check that the things we have are correct before emitting fire etc

[pookage]

As the functionality for the left / right button is duplicated, it might be worth moving their logic into a separate <blaster-controls-button> and just have this <blaster-controls> render them both, lay them out, and handle the window events.