ui: dragging a double-click highlights in word-sized chunks

Double-clicking selects a word. If you then drag from a held double-click, the behavior should be that it highlights text by extending to the next (or previous, depending on direction) word boundary. Currently, it just goes one rune at a time like a single-click drag.

This behavior is somewhat subtle, but other modern textbox implementations seem to do it, so we should consider it.