checkboxGroupInput: Adding "columns" control

[EricKrantz]

When the number of checkboxes grows large, the user has to scroll down the page to see them all. There is a complicated (to me) workaround on StackOverflow using css and HTML (https://stackoverflow.com/questions/29738975/how-to-align-a-group-of-checkboxgroupinput-in-r-shiny). Adding a column control would reduce the amount of css and HTML.

Of course, columns would be ignored if inline = TRUE.

Something like this (added in last row):

checkboxGroupInput(
  inputId,
  label,
  choices = NULL,
  selected = NULL,
  inline = FALSE,
  width = NULL,
  choiceNames = NULL,
  choiceValues = NULL,
  columns = 1
)

[cpsievert]

FWIW, we're planning on putting a more scalable version into the {bslib} package. It's very much a work-in-progress that likely won't be ready for at least a few weeks, but if you're keen to try it out: rstudio/bslib#484