Restore warning icons

So I think this sort of works to do what you're after, take a look at the changes here:
https://github.com/jamonation/dxr/commit/786d4fdeb1aae38304b8e9e4d54a00b592651dbd

The other stuff is just code highlighter changes, so ignore those (or pull from mozilla/dxr master).

Any thoughts about how to use the annotation icons? I've set it up so if there is more than one icon per line, on hover the rest will be displayed.

You have to click to display the warning which is a bit of a pain. But then using hover instead makes it hard to be able to select the actual annotation text since when you go to select it the warning disappears.

Is keeping the text visible so that you can copy/paste the note a useful thing?