FR: convert more things to normal links

[imax9000]

It's somewhat cumbersome that a lot of buttons and things that look like links aren't implemented as actual links and do not behave like links.

To list a few examples of things that don't open a new tab on middle-click/ctrl-click and don't allow copying the target URL from the right-click menu:

• Path elements in the path header
• File/directory entries in the file tree
• Line numbers in the main view/search results
• Path elements in the search results headers
• "Go to definition" and "Find references" buttons in code navigation popup

[brendan-kellam]

Going to use #519 to track this