Dropdown width independent of container width. Implements #1326

I've hit the need to have the select width as wide as the option is has selected (no extra white space to the right, if it's text is much smaller than the other options) but have the dropdown at the normal auto-calculated width.

An good example of this is the "Branch" select in github in which the select is much narrower than the actual dropdown. Actually most selects in github are like this: see the "Watch" and the "This repository" ones.