Do not blindly overwrite all files

At the moment the StatechartEditor overwrites all files when it saves something. This leads to annoying errors if you while the gui is open.

Correct behaviour:

• Do not touch unchanged files
• Only write files containing changes

Preferred behaviour:

• Do not touch unchanged files
• If a file was changed, copy the three files (changed in editor, original, changed on disk) in /tmp, start a three way merge with some tool (e.g. meld) and write the merge result.