Update "@theia" packages (extensions) list, to include interesting recent extensions

[marcdumais-work]

ATM, the example applications in this repo are very minimal and lacks some potentially interesting extensions like "@theia/secondary-window" (detachable windows) and probably more. 

It would be easy and good to align on Blueprint, for which Theia framework extensions we pull:

https://github.com/eclipse-theia/theia-blueprint/blob/master/applications/electron/package.json#L46-L85