[Tizen] Introduce AppcoreContext to make Tizen task switcher be able to control
xwalk's web applications.

Xwalk's web application must communicate with Tizen task switcher.
Otherwise, the task switcher kills the process.
For example, when switching from another application
to xwalk's web application, web application process should handle
the task switcher's request.

The most important task we need to do is to call appcore_init() in Tizen.
appcore_init() connects Tizen task manager via IPC. After calling
appcore_init(), the given process that called appcore_init() can communicate with
Tizen task switcher.

AppcoreContext calls appcore_init() with a callback function, and handles
messages in the callback function. Currently, AppcoreContext handles only
AE_TERMINATE message. When AppcoreContext receives AE_TERMINATE message,
AppcoreContext quits the xwalk. The task switcher sends AE_TERMINATE message
when an user kills the app on the task switcher.

AppcoreContextImpl uses private Tizen appcore API to reuse Tizen 2.0 implementation.
This private implementation of appcore is a bit huge. We will decide later
whether to maintain the code similar to appcore private implementation in xwalk
or not after investigating appcore in Tizen 3.0.

To concentrate Tizen implementation detail on cc file, we declare
AppcoreContextImpl that extends AppcoreContext in cc file.

NOTE: this implementation is not compatible with shared process mode, because
tizen task switcher cannot recognize multiple tasks per process. Shared process
mode requires one xwalk process to include multiple tasks (= web applications).
It is not supported by current task switcher.

appcore_context.cc is similar to a part of appcore-efl.c in appcore.

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