fix favicon routes for pages that are created during or after startup

[falkoschindler]

Until now favicons are collected in a global dictionary. During startup a function create_favicon_routes() is called to create routes for all favicons in this dictionary. This is because we need wait until ui.run(..., favicon=...) has been called because it might define a default favicon.

But this does not work for pages that are also created during startup (but a bit later) or even dynamically caused by a user event.

from nicegui import ui

ui.label('The favicon is correct on the index page.')

def startup():
    @ui.page('/sub')
    def page():
        ui.label('There is no favicon route for this page because it was only created during startup.')

ui.on_startup(startup)

ui.run(favicon='favicon.ico')

This PR proposes to create routes directly when initializing pages. Even though we need to wait for the favicon argument in ui.run, we can already create routes and let the callback access the fallback favicon when called. This simplifies things (no need for the global dictionary anymore) and works more robustly, since pages can be created at any time and favicons will be routed.

[rodja]

Very nice!