Add `can-shrink` property to GtkDropDown
There is no easy way to ellipsize the label in GtkDropDown. Since GtkButton and GtkMenuButton have a can-shrink property, I believe it would be appropriate for GtkDropDown as well, since it's designed to work with strings by default.
Sometimes you have dropdowns with dynamic strings and they can be very long, and then the dropdown ends up taking too much horizontal space. See below for an example of GtkDropDown becoming too long when selecting an item with a long string.