### clickable buttons
There's a facility for rendering buttons and responding to events when they're
clicked. It requires setting up 3 things:
  - a `state` table housing all buttons. Can be the same `state` variable the
    text-editor widget uses, but doesn't have to be.
  - specifying buttons to create in `state`. This must happen either directly
    or indirectly within `App.draw`.
  - responding to clicks on buttons in `state`. This must happen either
    directly or indirectly within `App.mousepressed`.
The following facilities help set these things up:
* Clear `state` at the start of each frame:
    ```
    state.button_handlers = {}
    ```
  Don't forget to do this, or your app will get slower over time.
* `button` creates a single button. The syntax is:
    ```
    button(state, name, {x=..., y=..., w=..., h=..., color={r,g,b},
      icon = function({x=..., y=..., w=..., h=...}) ... end,
      onpress1 = ...
    })
    ```
  Call this either directly or indirectly from `App.draw`. It will paint a
  rectangle to the screen with top-left at (x,y), dimensions w×h pixels in the
  specified `color`. It will then overlay any drawing instructions within
  `icon` atop it. The `icon` callback will receive a table containing the same
  x/y/w/h.

  The rectangle also registers within `state` the `onpress1` callback (without
  any arguments) when mouse button 1 is clicked on it. This way you can see
  everything about a button in one place. Create as many buttons as you like
  within a single shared `state`.
* `mouse_press_consumed_by_any_button_handler(state, x,y, mouse_button)`
  Call this either directly or indirectly from `App.mousepressed`. It will
  pass on a click to any button registered in `state`. It's also helpful to
  ensure clicks on a button don't have other effects, so I prefer the
  following boilerplate early in `mousepressed`:
    ```
    if mouse_press_consumed_by_any_button_handler(state, x,y, mouse_button) then
      return
    end
    ```