chromecast: start working on the chromecast receiver

(WIP)
chromecast: rework the loading indicator

It is now more similar to the one in Android.
chromecast: continue working on the chromecast receiver

However, newer apps have to use the new SDK. Instead of using SSDP +
DIAL, it now uses mDNS and some protocol on port 8009. Currently, it
doesn't seem anyone has tried something on this. See:
 https://plus.google.com/+SebastianMauer/posts/83hTniKEDwN
chromecast: add heartbeat mechanism to supervisor

This works like the Android version.
chromecast: remove debug code
chromecast: display status text to let know our status

This will come handy to know the state of the receiver without opening a
session with it. We will also be able to get updates without querying
anything.
receiver: abstract a bit the way we store data in receiver

The next step is to allow to retrieve data from the URL too.
receiver: send heartbeat only when we have a timeout set
receiver: fetch local storage from URL if available

This will allow to pre-register Chromecast devices.