ui: add 3-click search

3-clicking a non-directory searches for matching text in the body of the corresponding Sheet (or if in a Col background, just in the textbox itself). All matches are highlighted, but the cursor remains on the clicked match. On subsequent clicks, the cursor jumps to the next match. (This slightly deviates from Acme, which jumps to the next match even on the first click.)

Start by locating the UI handling for three-clicks and the representations of Sheets, Col backgrounds, directories, textboxes, and cursor movement. Trace how text selection and highlighting currently work, then verify the requested behavior: search the appropriate body or textbox, highlight all matches, keep the cursor on the first clicked match, and advance on subsequent clicks.

Written by the indexing model from the issue text.