The UIs aren't *any* UIs. They are specifically *machine* UIs. They need
to be able to fetch data from components on those machine entities,
which means they need some way to find the machine. Passing an entity ID
is the easiest way, IMO.
Added a `FuelChanged` event which is to be used to trigger Observers
watching for a machine's fuel level change.
I've written a slapdash observer to emit these events when the cutting
machine's big red button is pressed. That observer *must* be replaced
because of how it feeds the machine ID into the UI (it only works when
there is exactly 1 CuttingMachine, which won't be the case).
The dummy machines are missing their machine components. I've added the
`CuttingMachine` component to the cutting machine just to test the event
passing.
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.
