This removes the need to pass a lot of these `ParamSetter`s and
`GuiContext`s around. We also don't need explicit events to reset a
parameter anymore since you can get this information from the parameter
itself.
This will need to wait for https://github.com/vizia/vizia/issues/85 as
right now it's not yet possible to get this to behave correctly (other
than perhaps sending manual WindowEvents on the next frame).
