Skin Composer disallows special characters when naming resources

[misterbander]

When naming resources (fonts, colors, drawables etc.), Skin Composer disallows the use of special characters (@, #, !, spaces etc.), even though libGDX's skin has no restrictions on resource naming.

Tested on version 51

[raeleus]

Yes, this is a limitation I added myself. In retrospect, it was an unnecessary, arbitrary limitation. I'll have to fix this in the future.