Introduced different modes of execution:
SINGLE_THREAD - only one thread, the net is executed until no transition can be fired.
EVENT_LOOP - one additional thread, the event loop thread, where the Petri net runs and processes actions.
DETACHED - same as event loop, but the actions are deployed in a separate detached thread.
JOB_QUEUE - same as event loop, but there is a job queue thread where the actions are executed. The order of execution is preserved in this mode.
Added XML importing and exporting.
Multiple bugs fixed.