shiny-ui-config: Add new inputs and outputs for data files

* R/shiny-ui-config.R(mk_ui_config): New output fnDataFilesCtrl. New
  button impDataFilesB.

<shinyFiles observers>: New observer impDataFilesB

<restore-conf>: Do not modify the rv state for all conf
parameters. Instead, just do that for rv$m$conf$project. The rest will
be taken care of in <build-config>.

Also, update the new `datafiles' input.

<update-files-on-restore>: Add dependency on rv$m$input$tab$setid.

<build-config>: Update rv$m$conf$data from the new input.

<fnDataFilesCtrl>: New render.