Improve select popups

[rubenthoms]

When select elements are located at the end of a page / scrollable container, their popups are cut off. Either refactor the select component or find a different solution to this issue.

The idea is to stick to how the native select component works. Here are some screenshots of how it works including an evalutation if the behaviour can be recreated for custom popups using JavaScript.

1. select at end of scrollable container with enough space below - popup displays at top level, independent of scroll area:
   

• Can be recreated by appending the popup to body.

2. select at end of browser window - popup displays outside of browser window:
   

• Impossible to recreate this behaviour
• Possible workaround: display popup above custom select component

3. select at end of browser window while the latter is in full screen mode - popup displays above select component:
   

• Recreatable with same approach as in 2.

[rubenthoms]

Related to plotly/dash#2242.