I've taken a lot directly from the Bevy UI button example.
(https://bevy.org/examples/ui-user-interface/button/)
I'll make it look better later. For now, it just needs to exist. Onward
to the UI operation system!
I want the different "scenes" to be their own plugins for ease of setup
and reading.
The main menu plugin has been renamed to have "Plugin" first. This is so
the lexical sort in the docs places all the plugins next to each other.
The "get-ready" plugin has been given an empty struct and an
`impl Plugin` to match the main menu plugin. I've started the game over
scene, but left it unimplemented.
The non-gameplay scenes are really just a bunch of widgets. I'm going to
put them all together and then bundle the functionality with some
exported plugin builders.
