Separate the EGL platform handling from the rest of the libwebos-gui code.

From a concrete point of view, it means putting the OffscreenNativeWindow in a separate hybris eglplatform library.

* Create a new eglplatform-offscreen shared library
* Separate the window class and the buffer class
* Provide an interface IWebosEglWindow to manipulate the offscreen window from the QPA without knowing about the egl-platform involved.

Nota Bene: this PR belongs to the following set of PR for webOS-ports: libwebos-gui, qt-webos-plugin, webappmanager, luna-sysmgr and meta-webos-ports.