Different font for main window [CR]

Encapsulate the glyph cache (based on the truetype font) and the bitmap font into separate classes.

Use a json file for loading the font settings. Allow different font settings for the terrain window.

This can be used for testing: (put it into data/fontdata.json).
```
{
    "fontblending": false,
    "fontwidth": 8,
    "fontheight": 16,
    "fontsize": 16,
    "typeface": "FixedsysTTF",
    "map_fontwidth": 20,
    "map_fontheight": 20,
    "map_fontsize": 16,
    "map_typeface": "Terminus"
}
```

This is technically ready (that means it works correctly for me) but needs testing. Especially on non-linux platforms.

As always I'm open for suggestions/critics/requests.