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.