More API changes

[cpsievert]

Follow up of sorts to #51

• Define __all__ in shiny/__init__py to better define what from shiny import * does:
  1. The main benefit is that we won't clutter the global namespace with stuff that most apps won't need (e.g., submodules like reactcore, etc)
     2. In general, I think we should avoid listing submodules in __all__ and instead list either sub-packages (i.e., ui) or the the most important objects within submodules (e.g., App, render_plot, etc).
     a. The "down-side" is that you have to from shiny import foo to access a submodule, but by being explicit about it, it feels like a good thing considering (3):
     3. It seems very difficult for users to know what the "true" public-api for a submodule. It seems, by default, most dev environments will list all objects that don't have a _ prefix in that sub-module. For example, this is you get for a tab-completion experience on main right now when typing @reactive.[TAB]

• Define __all__ in every submodule (closes Need to define __all__ for each .py file #56)

  • Note this only impacts the result of from shiny.foo import *. It doesn't necessary discourage from shiny.foo import bar (unless bar is imported from another submodule -- at least by default with pylance)
  • For the reason above, we should also flag more objects as private with a _ prefix (there doesn't seem to be a way to say "this thing is internal to shiny, but fine to use across shiny's submodules", but I'd rather err on the side of discouraging users to use it)

• Moves all UI related functions (even "dynamic UI" things like notifications, progress, insert/remove) to the ui sub-package (see More API changes #69 (comment))