PDI-3748 - Multiple running PDI instances on the same machine can lead to corrupted .spoonrc and other vital files

Added mechanism to check if kettle home folder already in use. Based on
java.nio RandomAccessFile and locking.
Introduced new dialog to warn user.
Hope concurrently running instances of Spoon is rare case.
Added simple junit to check file lock implementation is working.
Due to junit tests restrictions - we can't simulate real 'between
processes on same platform' interactions.