Fix some minor issues

Fix: Resolve some small and minor (style) issues.
Local variables where it is possible

Fix: Using local variables instead of object ones where it is possible.
New last files list format

Add: The last files list uses ":::" instead of ";;;" as seperator.
Working directory in user.home

Add: Nodemapper uses {user.home}/.nodemapper as working directory (e.g. for storing the last files list).
Logging system

Add: An actual logging system which prints out log messages into {user.home}/.nodemapper/nodemapper.log.
Add: A message dialogue which shows up every time an error occures.
Update changelog with the logging system & home dir

Note: Update the changelog so it contains the new logging system and the new working directory in the user's home dir.