Define customizable <select>

Define several concepts related to customizable <select>:

- Relax the HTML parser to allow more tags inside <select>
- Update the content model to allow more elements inside <select> and related elements.
- Define rendering and some interactions for base appearance for <select>
- Add the <selectedcontent> HTML element
- Add a UA stylesheet for <select> with base appearance

Closes #9799. Closes #10310. Closes #10317. Closes #10520. Closes #10762.

The following pull requests are superseded: closes #10557; closes #10586; closes #10629; closes #10633; closes #10670.

Author: josepharhar