Select: Extend select with md-select-header directive

[benjamincharity]

This will allow an optional UI change that exposes a fixed search box at the top of the select allowing users to filter large collections.

This will be very close to the autocomplete so we will need usage docs. (use autocomplete when filtering happens on the server and filtered select when filtering happens on the client?)

Might be waiting on a merge angular/components#7835

• https://stackblitz.com/edit/mat-select-search?file=app%2Fmat-select-search%2Fmat-select-search.component.ts
• https://github.com/OasisDigital/angular-material-search-select/blob/master/src/app/search-select/base.ts
• https://github.com/bithost-gmbh/ngx-mat-select-search
• Feature request: search/filter in select angular/components#5697 (comment)

[macjohnny]

@benjamincharity The implementation here angular/components#5697 (comment) might be interesting for you.

[benjamincharity]

Currently on hold since ts-autocomplete is serving all current needs.

[benjamincharity]

closing in favor of #847