Replaced task window font setting with application font override.

Issue #105

I'm not sure why we have/had the option to change only the font size for the
Task Window, this patch allows you to override the application default font
with your own selection. The tracker seems to ignore the system font, but I'm
guessing it's due to it needing a small font in order to fit all that
information in.

Gave this a shot, I haven't played around with the Charm configuration stuff
much so I'm hoping I didn't do something that might make Frank cringe. ;)

Screenshot: https://imagebin.kdab.com/src/1364796370783.png