Add feature to export blocks as image.

[icarito]

This feature is good for writing documentation.

[icarito]

The export palette is becoming crowded, in my 768 height screen it appears on the left. This looks like an issue with the gtk2 toolkit. It does work nevertheless.

[icarito]

After further testing this, I would like to make 2 improvements when I find the time:

• [ ] self.canvas.height is not the same as the programming canvas - long programs get cropped.
• [ ] autocropping - For my own use, locally, I used ImageMagick magic mogrify -trim command but to avoid dependency a cairo solution would be better.

I'm using this to finish https://icarito.gitbooks.io/physics-with-xo/ .