Make better use of existing shiny classes

[jankowtf]

Better understand the interaction between the classes shiny::ReactiveValues, shiny::Observable and shiny::Context with respect to invalidation (fields .invalidated and associated functions).

As currently only a modified version of shiny::Observable is used, there still needs to be this manual distinction to be made in setShinyReactive():

if (o$.invalidated) {
  out <- o$getValue()
} else {
  out <- o$.updateValue()
}

This feels wrong and might also lead to inconsistencies when trying to integrate reactive objects set by setShinyReactive() in the shiny workflow.