Use an abstract IWebosEglWindow interface to manipulate the EGL window.

This interface is defined in libwebos-gui, so the dependancy to libhybris itself is not needed anymore.

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 and qt5-webos-plugin.