WIP - handle Leaflet.draw add/delete with R leaflet layerManger

[timelyportfolio]

#96

Questions:

• R leaflet layerManager only supports full removal with the key combination of category and layerId. I used category as null but we could easily use something like draw or even the same as group. For layerId, I just used String(L.stamp(layer)). I would make pull to R leaflet for other removal methods, but I know that acceptance might be slow.
• How might we assign a more knowable layerId to support removal of single layers? I know that a user could monitor the Leaflet.draw events in Shiny to keep a list, but is there an easier way.
• How could this be used if we are editing existing features? In this case the above might be much easier, since a user should know what layerId was assigned.

Code to demonstrate:

#devtools::install_github("timelyportfolio/leaflet.extras@draw-layermgr")

library(leaflet)
library(leaflet.extras)

leaflet() %>%
  addDrawToolbar(editOptions=editToolbarOptions())

library(shiny)
ui <- tagList(
  leafletOutput("leaf"),
  actionButton("btnclear", "clear drawn")
)
server <- function(input, output, session) {
  output$leaf <- renderLeaflet({
    leaflet() %>%
      addDrawToolbar(editOptions=editToolbarOptions())
  })
  
  observeEvent(
    input$btnclear,
    {
      leafletProxy("leaf") %>%
       clearGroup("editableFeatureGroup")
    }
  )
}
shinyApp(ui,server)

[bhaskarvk]

I need to review this but looks like a good starting point. I can get to this over the weekend.