More colourful graphics

[Andy Balaam]

Pull request #569 inspired us to make the graphics more colourful!

Start by making earth blocks brown (with grass) like this:

but without grass growing underground. Include sloping blocks.

In your development work, keep the grass graphics separate because later we want to draw them on top of the rabbits.

Once that is done, close this issue and make more for things like:

• Add flowers like in the first screenshot of #569
• Look into drawing some graphics on a top layer (in front of the rabbits) - this would include grass and flowers
• Consider new graphics for things like exits and entrances. Look on openclipart and other sources of graphics compatible with our license
• Add skeletons, buried treasure etc. in some earth blocks
• Improve the look of metal blocks
• Consider allowing choosing a theme for each level, in a similar way to the way we choose music now. Allow themes to be partial, so theme authors do not need to re-draw every asset. Consider allowing rabbits and rabbots to be themed too.