Gimp Plugin: A better lazy man's city?

[Alfar]

A new version, with support for undo (Thanks for the tip RobA, ended up working on a copy, works great, probably even faster than before)

Added a courtyards option, which will make only one layer of houses, and cut out the middle. It makes for much more regularly shaped houses as well.

Added a delete invisible layers option (which doesn't actually delete anything, just doesn't copy them back over from the copy image)

Added a merge layers option, which (duh) merges the visible layers before moving them to the original image.

Also cleaned up some of the code to use constants rather than integers as advised by RobA. I know it's a good idea, I just had some trouble converting the constants from the documentation to their names in python at an early point when learning to write plugins and sorta gave up. Mostly all is good now, though, so will clean up the rest of the code in time.

Now for some jittering