Support multiple slots

[malmaud]

Is your feature request related to a problem? Please describe.
I want to define content components that accept multiple arbitrary components. eg:

• if i wanted to define a tab view where the user can pass in multiple arbitrary components for the different tabs
• or a 'content with header' component where the content and header are each arbitrary user-supplied components

Describe the solution you'd like
Some way to pass arbitrary components as children or parameters to content components, not sure what the API should look like. Maybe something like

with header_and_body_component() as parent:
   with parent.header():
       me.text("my header")
   with parent.body():
       me.text("my body")