manually maintain mouse button press state

manually maintain mouse button press state

Just checking mouse.isDown works if the editor is the entirety of the
app, as is true in this fork. However, we often want to introduce other
widgets. We'd like tapping on them to not cause the selection to flash:
  https://news.ycombinator.com/context?id=38404923&submission=38397715

The right architecture to enforce this is: have each layer of the UI
maintain its own state machine between mouse_press and mouse_release
events. And only check the state machine in the next level down rather
than lower layers or the bottommost layer of raw LÖVE.