Integrate DuckDuckGo Instant Answers

[kdwk]

With other operating systems like Windows and macOS, there seems to be a universal search menu for applications, files and other information. I was wondering if the elementary applications-menu can be transformed into something more useful like that -- a central place to do all searches, kind of like Spotlight in macOS.

To be clear, I am not suggesting that we enable web search from applications-menu, but to use the DuckDuckGo instant answers API (https://duckduckgo.com/api) to get useful information like word/ phrases definitions, get Wikipedia answers to questions like 'how tall is Barack Obama' and 'how far is the Moon', and stuff like that.

This fits in well with the project to replace ZeitGeist with Tracker. If we show files and stuff like that, we might as well show instant answers as well.