Busy indicator improvements

[cpsievert]

Closes #4169
Closes #4115

This PR makes the following improvements to useBusyIndicators():

• Shows a spinner on recalculating htmlwidgets that have previously rendered an error (including req() and validate()).
• Shows a spinner on tableOutput().
• Places a minimum height on recalculating outputs so that the spinner is always visible.

Testing app

library(shiny)
library(reactable)
library(plotly)
library(visNetwork)

ui <- fluidPage(
  useBusyIndicators(),
  actionButton("run_tbl", "Run table"),
  actionButton("run_reactable", "Run reactable"),
  actionButton("run_text", "Run text"),
  actionButton("run_plotly", "Run plotly"),
  actionButton("run_network", "Run network"),
  textOutput("text"),
  fluidRow(
    column(6, tableOutput("tbl")),
    column(6, reactableOutput("r_tbl"))
  ),
  plotlyOutput("p"),
  visNetworkOutput("network")
)

server <- function(input, output, session) {
  output$text <- renderText({
    req(input$run_text)
    Sys.sleep(3)
    "test"
  })

  output$tbl <- renderTable({
    req(input$run_tbl)
    Sys.sleep(3)
    head(mtcars, 10)
  })

  output$r_tbl <- renderReactable({
    req(input$run_reactable)
    Sys.sleep(3)
    reactable(mtcars)
  })

  output$p <- renderPlotly({
    # validate(need(input$run_plotly %% 2, "Please run plotly"))
    req(input$run_plotly)
    Sys.sleep(3)
    plotly::plot_ly()
  })

  output$network <- renderVisNetwork({
    # validate(need(input$run_network %% 2, "Please run network"))
    req(input$run_network)
    Sys.sleep(30)
    nodes <- data.frame(id = 1:3)
    edges <- data.frame(from = c(1, 2), to = c(1, 3))

    visNetwork(nodes, edges)
  })
}

shinyApp(ui, server)