Refactor engine class
The engine class has accumulated a lot of stuff, that actually belongs into the various mw*-subsystems.
Issue #19: factored out local script handling into a separate class
This also fixes a bug related to self-destructing references (introduced during the cell handling improvements)
Issue #19: Focus Reporting Improvements
- moved focus reporting code out of the framelistener
- made focus reporting optional (new --report-focus switch)
- report based on tiem passed instead of number of frames passed
- only report if focus has changed
Issue #19: Create OGRE renderer in go function instead of Engine's constructor
This avoids setting up a renderer each time openmw is called with the --help switch.