[Tizen] Introduce TizenAppcoreWrapper to make Tizen task switcher 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.

All we need to do is to call appcore_init() in Tizen. After calling
appcore_init(), the given process that calls appcore_init() can communicate with
Tizen task switcher.

NOTE: this implementation is not compatible with shared process mode,
because tizen task switcher can recognize each task per process.
Shared process mode requires one xwalk process to include
multi tasks (= web applications).
It is not supported by current task switcher.
Shared process mode requires the change of tizen application framework.

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

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