More configurable theme; start on Bootstrap 4 support

[cpsievert]

This PR adds a new option, radiant.theme, which makes it possible to set a custom theme (without needing to know about the other defaults behind radiant.nav_ui).

The other CSS changes make things look slightly better when using Bootstrap 4 via the new {bslib} package. See also radiant-rstats/radiant.basics#2 which adds better support for Bootstrap 4 in {radiant.basics}.

library(radiant)
# remotes::install_github("rstudio/bslib")
opts <- options(radiant.theme = bslib::bs_theme(version = 4))
shiny::onStop(function() options(opts))
shiny::shinyAppDir(system.file("app", package = "radiant"))

[vnijs]

This looks really interesting @cpsievert! I'll make time to review asap. I did notice that bslab is not yet on CRAN. Is there a (rough) timeline for that?

[vnijs]

I just tried this out locally and I'm not seeing any differences. I also tried the my_theme example from the bslib page and see no differences when starting radiant.data. I installed bslib and updated htmltools to the github version as required. Seems I'm missing something.

[vnijs]

Seems like this also requires the development version of shiny. the my_theme example looks great! The main issue I see is that navbar-right is being wrapped to the next line for some reason. Also happens with basic bs4.

There are few other minor issues in basic bs4 that I see so far. See below paging for the DT table and the "store" button taking up two lines. I assume giving the button a bit more space (or reducing size of the +) would address the 2nd issue. Not sure about DT paging though.

[cpsievert]

Good catches, thanks. cd88f5d solves the DT issues. I don't have a great solution in mind for the "+ Store" button (you could add back in the white-space: nowrap that BS3->BS4 removes on the .btn class, but that'll make the label on the input span 2 lines...it seems a "real" fix might be rather involved without a bunch of benefit, so I'd suggest leaving it)

As for the navbar issue, this is what I see when I run shiny::runApp(system.file("app", package = "radiant"), launch.browser = TRUE) (without bslib), is the expected result?

[vnijs]

Thanks for the update and suggestions. I'll look into this next weekend. I think the change in with navbar-right might have been introduced in the dev version of shiny. Below what it should look like with shiny 1.5.0. FYI navbar-ight is used to indicate if you are using an Rstudio project or if not what the base directory is that Radiant will use.

[cpsievert]

Ah, yes, code like this is always gonna be susceptible to changes in shiny:

radiant.data/inst/app/global.R

Lines 401 to 405 in b57c25e

	## based on: https://stackoverflow.com/a/40755608/1974918
	navbar[[3]][[1]]$children[[1]]$children[[2]] <- htmltools::tagAppendChild(
	navbar[[3]][[1]]$children[[1]]$children[[2]],
	proj
	)

I think {shiny} can do better to not break this code if {bslib} isn't being used, but I'll send you some changes to better accommodate {bslib} once rstudio/shiny#3235 is closed

[vnijs]

@cpsievert Thanks again for your help transitioning. Closing.