add fallback font

This bundles Comic Neue with the executable and sets it as a fallback font. On systems where Comic Sans is available this commit changes nothing and Comic Sans is still used. On systems without Comic Sans, with this commit Comic Neue is used instead.

In overview, Qt selects fonts in the following order (first available is used):

Before: Comic Sans, Helvetica, lastResortFont (any common font) 
With this PR: Comic Sans, Comic Neue (bundled, so always available)

Comic Neue is not a perfect replacement (notably it's thinner than Comic Sans), but it's a better fallback than Helvetica.

This fixes #508