Deprecate App struct in favor of function based interface

[jstarry]

Since we no longer really need the App struct at all, have you thought about deprecating it in favour of a more function-based interface?

// old 
App::<MyComp>::new().mount_with_props(target, props)

// new
yew::mount_with_props::<MyComp>(target, props)

This might even bring the API closer together with yew::start_app and the like.

Originally posted by @siku2 in #1646 (comment)

[nicklaswj]

I agree that App currently have no use and it can just as well can be replaced with a function based interface.
HOWEVER: I've made a pull request that actually puts it to use again, and I think the functionality that the PR adds semantically fit in the App struct. #1825

[ranile]

Didn't #1825 make the required changes? I think this issue should be closed now.