Introduce ApplicationEventPropagator to integrate XWalk on each platform.

XWalk needs to receive platform callbacks related to application lifecycle and
device status.

This patch introduces ApplicationEventPropagator to register XWalk to the
application framework of each platform. This patch introduces
ApplicationEventHandler interface that will receive platform callbacks.

XWalk needs to implement ApplicationEventHandler and register it to
ApplicationEventPropagator, and then XWalk can receive following callbacks:
Exit, Pause, Resume, MemoryPressure, CriticalDischarge, and OrientationChanged.

Each platform should implement ApplicationEventPropagator.

Runtime implements ApplicationEventHandler. This patch handles only exit, pause,
and resume. We will handles other callbacks in the future. I referred to android
code (content_view_core_impl.cc) to handle pause and resume.

BUG=https://github.com/otcshare/crosswalk/issues/609
[Tizen] Integrate XWalk on Tizen application framework.

After this patch, XWalk doesn't crash when an user switchs a web application
using Tizen task switcher.

Implement ApplicationEventPropagator for Tizen platform.
Implement a subset of appcore in XWalk as we just need.

This implementation is similar to Tizen appcore implementation. There is a Tizen
file which each xwalk file plays a similar role.
1. application_event_propagator_impl.cc is similar to app_main.c in application
library.
2. tizen_app_lifecycle_manager.cc is similar to a part of appcore-efl.c in
appcore.

BUG=https://github.com/otcshare/crosswalk/issues/609
[Tizen] Support Pause and Resume callbacks.

Implement Pause and Resume callbacks using DesktopRootWindowHost.
When all windows are invisible, Pause event is emitted. When at least one window
is visible, Resume event is emitted.

Tizen application framework implements Pause & Resume in the same manner. You
can see the same logic in appcore-efl.c in appcore.

Surprisingly, appcore does not notify Pause & Resume although appcore looks like
being able to handle Pause & Resume. I also tested appcore-efl, and all Pause &
Resume events is originated from x events, not appcore (specifically vconf).

BUG=https://github.com/otcshare/crosswalk/issues/609
[Tizen] Support DeviceOrientationChanged callback.

Implement DeviceOrientationChanged supporting using Tizne sensor.

Tizen application framework implements it in the same manner. You can see the
same logic in appcore-rotation.c in appcore.

BUG=https://github.com/otcshare/crosswalk/issues/609