Updates for biomed

I have made some improvements to biomed, my biome editor for Minecraft maps:

• There now is a editor for the biome presets (Screenshot), so people who use mods for Minecraft can add their biomes.
• Some menu items now use stock icons and key bindings. Gives the whole thing a better Look&Feel.

For these changes I needed some GTK features that were not included with go-gtk, so I implemented them.