Fixed stray use of old BOSS directory var.
BOSS launcher improvements.

* Fixed BOSS registry path overriding local install path if both exist.
* Fixed GUI being run by default.
* GUI launch option is now a checkbox menu item, rather than a separate
launcher.
* Removed some custom launcher code in favour of using the generic
inherited code.
* Added a wait argument to the generic launcher code that causes Bash to
wait for the launched (exe) process to finish.
Merge pull request #128 from wrye-bash/boss-launcher-fix

BOSS launcher fixes