Mount API proposal

# Mount at start
self.mount(Foo())
self.mount(Foo(), before=0)

# Mount before first Foo
self.mount(Foo(), before="Foo")

# Equivalent to...
self.mount(Foo(), before=self.query_one("Foo"))

# Mount multiple widgets
self.mount(Foo(), Bar(), Baz())

# Mount at end
self.mount(Foo())
self.mount(Foo(), after=-1)

# Add widget after another widget
self.mount(Foo(), after="#bar")

There will be other options once we implement :first and :last psuedo-selectors

# Mount before first Foo
self.mount(Foo(), before="Foo:first")

# Mount after last foo
self.mount(Foo(), after="Foo:last")

Thinking about selectors. Calling self.query would return grandchildren. It would be reasonable to expect that to do the same in mount. I think that a selector, i.e. self.mount(Foo(), before="Foo") should effectively do a query_one and mount before that widget, which may not be an immediate child.

Here's the equivalent JS API:

https://developer.mozilla.org/en-US/docs/Web/API/Node/appendChild

There's a ton of methods which do much the same as the proposed mount.