Changes to PGen for better user experience

1. Add apply with enter key when user wants to edit an edge properties. This means user can press enter key to apply changes of edge like pressing apply button.
2. When user saves or opens a file once, in other savings or openings of files the last path that user had chosen, can be remembered.
3. When user press Check Graphs, if an edge has no token, in PGen the error was shown only with "Token can not be empty" message. Replace it with better error with giving start and end node of the edge that has no token.
4. Sometimes users may forget save files and press close button of program accidentally. For this problem, we will warn user whether he/she wants to save the program or not.