Normally this would be a compilation error, but Bevy has an
`impl Bundle for ()` somewhere.
The intended bundle content gets discarded and a unit is implicitly
returned. That unit is implicitly converted into a `Bundle`, satisfying
the compiler.
The `BigRedButton` should be a real struct and component so that I can
query for it later.
I'm following the same API pattern as the `CloseButton` struct, in that
there is a spawner using the `Commands` (or `ChildSpawnerCommands`) and
the observer functions are static methods.
That was much easier than I expected it to be. I'm still going to alter
how the `machine_ui_base` function works so that I don't have to
remember to attach a button all the time.
I was originally planning to search up the entity hierarchy to find the
top-most entity and despawn that. This will become a problem if I ever
want to have panels in panels.
Instead, just record the target entity and despawn it.
This means the current usage doesn't work, so I've removed it from the
base machine UI bundle.
I'm trying out a new scope strategy. The CloseButton exists as a real
struct with the `Component` trait. There's a bundle spawning utility
function and several observer systems. The observer systems have been
added directly to the app, which I *believe* means they won't be
duplicated the way the Big Red Button's observers will.
