Now supporting Firefox(OS) webapps

What I did there I generalised most of the function calls from `tab` to `actor`, since both `tabs` and `apps` have a url. 

While for tabs one can just do `listTabs`, for apps we have to first get webapps actor `getWebapps`, `listRunningApp` and finally `getApp` actor.

Only thing missing is to get the current app opened instead of default gaia.
Don't use the term "actor". Actors are on the server-side, and this is the client-side so we're abstracting over that. Some options, open to others: "page", "app"