Add animbrush functionality + sprite sheets

[mriale]

• Grab animation frames as a brush
• Load and save animbrushes
• grab sprite sheets as animbrushes
• generate sprite sheets from animbrushes

[Sleuthgame]

Ambitious !

I'm trying to find some examples.

[mriale]

Many good and bad examples here: https://www.spriters-resource.com/

[Sleuthgame]

Some animbrush as animated icons here Not very funny.

[mriale]

Also add support for fonts in sprite sheet format. This should work with the text tool.

[Sleuthgame]

Lot of fonts in sprite sheet here : https://github.com/ianhan/BitmapFonts.