[SysApps] Add a stub for the Device Capabilities API

This patch adds the basic bits for starting the development of the
Device Capabilties API:

- The IDL files to generate the data serializers.
- The JavaScript shim file, initializing the internal extension system
- An Extension class and an Instance class.
- Register the extension on the XWalkBrowserMainParts (only desktop ATM).
[SysApps] Add an data provider abstraction for the CPU information

The data provider will access the underlying platform and fill a SystemCPU
struct. We are using Chromium classes to retrieve this information, which is
platform independent. In the following patch we will be adding a platform
specialization for querying the CPU load.

Note that the SystemCPU struct is not defined anywhere. By adding SystemCPU dictionary
interface to the IDL file, we instruct the generators to generate the header and
code for this struct, also a function to serialize the struct as a message we can send to
the JavaScript shim.
[SysApps] Add platform dependent CPU info providers

The buildsystem will handle the compilation units accordly to the suffix. No
compile time guards (#ifdefs) are necessary here.

The Windows implementation is just a no-op stub.
[SysApps] Add the Device Capabilities Object

Usually, we could be handling static methods at the Instance object, but this
API requires an EventTarget (coming soon) which adds a bit of complexity,
that's why we need an Device Capabilities Object.

DeviceCapabilitiesInstance has now a BindingObjectStore, which is a class for
keeping track of BindingObjects lifecycle and routing messages. It will
automatically destroy BindingObjects when they are no longer used and route
messages to objects based on the unique ID.

In the JavaScript side we have:

common.BindingObject.call(this, object_id) will initialize the BindingObject
and stablish the "link" for routing the messages.

DeviceCapabilities  is being exported at the root of the API namespace and cannot
be created with "new". We have only on instance of this object per app.
[SysApps] Implement the 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.

In the JavaScript shim, is important to use "this_.addMethodWithPromise" so it will
wrap the Binding Object routing ID automatically to the message and associate it
to the Promise.
[SysApps] Add CPUInfoProvider unit tests

The test basically pings each entry point of the API and check if
the returns values are within a sane range.
[SysApps] Add browsertests for DeviceCapabilities getCPUInfo()

Not a replacement for W3C conformance tests, just checking the basic API
functionality and sanity. The test is crafted with the addition of tests
for the rest of the DeviceCapabilities API in mind, that is why we have
a test list with just one test.