These fixes are meant to help for #1009

1. Fix bad fallback in eval_expr(). When eval_expr() failed it tried again, but stripped the last character. This would help to show for example "6" as the result when typing "6 *", until you type a second number. But the fallback was wasteful and applied too wide. I replaced it with a much more specific regex replace that strips the last operator (including ** which is one operator, but not ++ which is two)
2. I also added parenthesis completion for unfinished queries while I was at it.
3. Added memoization to avoid running the same evaluation repeatedly (should work especially well with the normalization)
4. Round the result to at most 15 decimals (as in Extend calculator functionality #1009, but applied consistently and doesn't zero-pad results)