[SysApps] Add a wrapper to make Promise work with Internal Extensions callbacks

Promises are a really nice fit with the Internal Extensions callbacks, because
they are single shot (you fire it once and forget about it).

We just need to create a wrapper around it with a "protocol" so we can
properly call fulfill() or reject() depending on the message we get from
the native backend.

The Internal Extension callback system will handle keep tracking of the mapping
between callback handlers in the native side and pending Promises in the
JavaScript shim automatically.
[SysApps] Implement Device Capabilities getCPUInfo()

We first add an callback and function to the IDL to make it generate the Promise
reply serializers, which contains the SystemCPU + error message if applicable.

A function handler for "getCPUInfo" is also needed and it will associate the
message coming from the extension with a class method. It is being registered in
the Instance constructor.

GetCPUInfo::Results::Create is generated from the IDL. This function will
create a message which the promise wrapper will receive.

info->PostResult() has internally the callback ID that maps to a callback ID in
the JavaScript shim, so the message will reach the correct recipient
automatically.

In the JavaScript shim, is important to use "internal.postMessage" so it will
use the Internal Extension callback system. The "[]" stands for the getCPUInfo
arguments, which is empty in this case.