Pass express' sidebar title arg to the page title

[cpsievert]

With this change, this code will yield the following:

from shiny.express import layout

with layout.sidebar(title="My title"):
    "Sidebar"

"Main content"

Instead of

(The latter design is intended more for "inline" sidebar layouts, which with layout.sidebar() doesn't support)

[cpsievert]

Closing this in favor of another (incoming) PR