I haven't touched Jtool (the source code, anyway) for 2 months, but the past few days I worked quite a bit on it and added a few new features, so here's version 1.2.0.
The download link is in the original post. Here's the change log:
--- 1.2.0 2/1/16 ---
Easier-to-parse map format
Rearrange depth of blocks, spikes, water
Easily erase objects beneath water
Killer / solid border map property
Check for new version on startup
Show filename in window caption
Include brown block skin
Fix rare map corruption bug
Lower death sound volume
A few misc tweaks
One new easter egg