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.
There are many better ways of doing this. Are you sure you can't just check for an empty `QVariant` with `isNull` or `isValid` instead? Or you can simply use `settings.contains` to check if the value is there.
In addition, you can pass a parameter to `value` that is used as the default returned if the setting doesn't exist, but I guess the point of this is so that the default gets written to the config value.