Stacks now have an alignment property, which allows them to align their
content.
Horizontal stacks can be aligned Top Bottom Center.
Vertical stacks can be aligned Left Right Center.
This is respected for both Stack::insert and Stack::component.
Alignment expects Stack::insert to contain a single line string, which
is a functional requirement for both Stack::insert and ViewContext::insert.
App now has a renderer object that can be retrieved to send a render
signal from outside the application.
This is useful for rendering based on operations taking place in other
threads.
Joe Bellus