Add a helper for setting page arguments

[cpsievert]

As highlighted in #852, this unfortunate code is required to get a page title with the sidebar layout.

from shiny.express import layout

layout.set_page(layout.page_sidebar(title="Page title"))

And we have a similar problem if you want to change page-level settings like gap, padding, lang, etc.

I think it could make sense to have both a .set_title() and a lower-level .set_page_parameters()

[wch]

Some options and comments:

1. layout.set_page_sidebar(title="Page title"): One would never actually use with layout.page_sidebar(title="Page title"), so it might make sense to get rid of that function and replace it with layout.set_page_sidebar(title="Page title") (and same for the other page functions). Drawbacks: the user still needs to know what kind of page function is used by default.
2. layout.set_title("Page title"): This is nice and simple, but doesn't really make sense to extend to all the arguments for page_xx functions.
3. layout.set_page_parameters(title="Page title"): This is more general than .set_title(), but we can't offer the user much support for knowing what parameters are available. So in the end, other than very common parameters like title, the user still would need to know which page_xx function they are using, and look up the available parameters there.

To me, there's not an obvious winner here.

[cpsievert]

Oh yea, I think I like having both 1 and 2. Also, I think we can probably have .set_title() do something sensible for all pages

[cpsievert]

We now have shiny.express.ui.page_opts() which gets at the main idea here