Offload time consuming operations to a WorkerScript.
Re-implement YouTube video URL fetching code in C++.

This gets rid of last traces of old JavaScript networking code. The new
implementation should also make it possible to implement video quality
selection.
NativeUtil: Cache region code

The region code value is determined based on current mobile country code
read from the phone modem. It's extremely unlikely it's going to change
during application runtime so cache it to avoid the extra dbus method
calls.