Fixes for shiny.express.ui.card()

[cpsievert]

Closes #948

[wch]

This code reaches into context-manager children in a way that's very specific to this one use case and also feels a bit brittle. I think this actually points to a more general issue that we're going to have to deal with.

Currently, in Shiny Core we have two stages of dealing with the tree of components:

• Building the tree with nested function calls. At this point, the tree may consist of strings, Tags, and Tagifiable objects (and a few other types which I'll ignore for this discussion).
• Calling .tagify() on the top-level node recurses through the tree and converts all nodes to strings, and Tags.

With Shiny Express, there's another type of object that can be in the tree: RecallContextManagers. These are actually Tagifiable, but the problem is that some functions, like card(), want to check if their children are a specific class, like CardItem, before deciding what to do with them. This happens before the .tagify() step. I think we'll need a generalizable way of handling RecallContextManagers, because this issue will likely crop up in other places.

Maybe the steps should be something like this:

• Build the tree with strings, Tags, Tagifiables, and RecallContextManagers. As the tree is being built, functions can
  check if their children are RecallContextManagers, and if so, call a method.resolve(), which looks like this:

  def resolve(self):
      return self.fn(*self.args, **self.kwargs)

  This would only be necessary to do in functions like card_body() which check if their children are objects of specific classes (that are not Tag).

[wch]

Closing because this was superseded by #992.