Release 3.8

[prmr]

• [x] Make sure all other issues for the milestone are closed;
• [x] Clean up any small code problem
• [x] Generate a fresh user guide using the tip generator
• [x] Run all the unit tests;
• [x] Increment the version number in JetUML.java;
• [x] Update the copyrights using the copyright tool;
• [x] Update the documentation;
• [x] Create a new release;
• [x] Write the release notes;
• [x] Release the application on GitHub;
• [x] Create a thin jar that only includes the source code (excluding tests) and the license;
• [x] Package the application as above for the three different platforms, and upload to the release
• [x] Compute SHA values and sign the binaries
• [x] Close the milestone;
• [x] Delete stale branches