Fixed bug when clicking on no tab in tab_manager.
Added task place to all PNs.

-> Added the task place and arc after the initialize method. (Added self._main_transition member)
-> Modified validation to allow NPTPlace to be connected to Preconditions and Rule Transitions.
-> Added task property getter and setter to petri nets.
-> Added the method set_task_name to PNEditors.
Fixed rename task method.

-> Modified delete_petri_net and open_petri_net methods to be used with rename.
-> Added the set_task_name function call to every petri_net copied when renaming.
-> Added REGEX to task name validation when renaming.
-> Fixed is_primitive_action tag bug.