Changed debugger font to Courier, added customization option

A monospace font would allow for adresses and other debugging information to be properly aligned. Fixes issue #67.
Also, we should use [this](http://qt-project.org/doc/qt-4.8/qcoreapplication.html#organizationName-prop) et al to set the organization and application name once instead of duplicating it in all `QSettings` constructors.