Attach a fallback font to the FontGroup list

[burg]

We should have a fallback font that's able to render almost all characters. It should be attached to the end of a FontGroup.

CSS3 Fonts, Section 5, Step 6 describes a "system font fallback procedure", which seems to mean "do anything reasonable". For now we could use Helvetica.

[aydinkim]

d7f4834

I got rid of fall back font - JosefinSans.
But according to this issue, we need add something to protect from program abortion due to lack of usable fonts.
Just we need to substitute JosefonSans to Helvetica?
or we need to attach Helvetica into FontGroup?

I will re-issue this.