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.
Change tho `index.column() == 0 || index.column() == 1`, please. The logic here is that we want to enable the font for columns 0 and 1, instead of "for everything else than 2". The suggested change is more robust against potential addition of new columns.