ui: Improve autocompletion

[beorn7]

We had iterated quite a bit on autocompletion in the expression browser, see #907 and #1181 . However, we reverted them all in #1303 because of the problems reported in #1185 . We never came back to it, but we should. Problem so far was that nobody felt really qualified about those UI intricacies.

@davkal (who asked for this issue).

[biglovisa]

Is this related to #3187, improving the matching algorithm for the expression browser? I was just touching that code and was hoping to find some time to take a look at it.

[davkal]

At the minimum, I believe this issue was to make the suggester work mid-query, not just at the beginning, e.g., for queries like go_gc_duration_seconds + go, where you dont get suggestions for the second go prefix. #3187 is more about the weighting (or lack thereof) in the fuzzy matching algorithm.