From what I can tell most of the functionally of this program is already possible in KDE4. While this takes a 'type what you want to do' approach, that is too much like a CLI to me but that is just my opinion. However for doing things like launching programs K-runner or Katapult do everything this program does in that regard. As for minimizing and maximizing windows and such, I would think clicking the min/max buttons are easier. But having a calculator that is easy to access, KDE could use something like this. However I think handling this the same way spell-checking is handled would work best considering from what I have noticed the times you need a simple calculator is when your typing text, otherwise it makes more since to just open a full-blown calculator application.
So similar to the way you correct a misspelled word, you could correct an equation. Say I write "1+1=5" obviously that is incorrect so it could be treated like a misspelled word; highlight it red or something and give options to correct it, like "1+1=2" or "2" in this case. Similar to the grammatical error meaning not the words but the assortment of them is in error, when I write "5+5" this could be considered the equivalent of a grammatical error and offer a complete equation or just the answer like "5+5=10" or "10". This would act like a calculator for general purposes and if it was set up the same way the KDE4 spell-checker is it could work anywhere you could type. Seem practical?