Simpler API to display/update inputs

[FrancoisGuillem]

For now, user has to use parameters .display and .updateInputs which are a bit complicated to use. A simpler API would be to pass directly expressions in the input generator function. For instance the following code:

manipulateWidget(
  plotCar(car),
  cylinders = mwSelect(c("4", "6", "8")),
  car = mwSelect(),
  .updateInputs = list(
    car = list(choices = carChoices[[cylinders]])
  )
)

could be replaced by:

manipulateWidget(
  plotCar(car),
  cylinders = mwSelect(c("4", "6", "8")),
  car = mwSelect(choices = carChoices[[cylinders]]),
)
``