Support icons in mat-select trigger

[calebegg]

Right now icons can be used in mat-option elements inside mat-select, but they don't show up on the trigger.

I know I can do this myself with mat-select-trigger, but it's complicated because it means referencing the logic for what to show both from the option and the trigger, and the default styling with an icon in the trigger is pretty bad; it expands the whole mat-form-field's height, and it's not easy to work around.

[crisbeto]

The feature makes sense to me, but my only concern is that it might become repetitive if you wanted to change the icon for all of the selects in an app. We can't add an injection token for it without introducing a dependency between material/select and material/icon.