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 now is a single vector with all of the information in the correct
order instead of the hashmaps and a vector. This avoids deduplication,
and it especially makes manual `Params` implementations a lot more
convenient since you can't mess up with mismatching IDs between the
methods.
To accommodate exactly this, the persistent fields methods also have a
default implementation and the trait has been marked as `unsafe` since
it's the programmer's responsibility to make sure these `ParamPtr`s will
remain valid.
This may hurt performance in generic UIs a bit, but it will allow you to
programatically generate custom Params implementations for repeated
Parameters structs.
