Improve compatibility when running on Linux devices

Several issues need to be resolved in order to improve compatibility on Linux-based devices:

[Bug] ~The user-selected serial port is reset by the background port update function. Fix this to ensure that the selected serial port is retained.~ Fix implemented
[Bug] ~When connecting to a serial port, the first SerialEvent which is triggered results in a NULL object string. Put in a check for null in the SerialEvent function to prevent an error message from appearing.~ Fix implemented
[Privilege Issue] Linux requires elevated privileges in order to connect to the serial comm ports. If the program is not run using the "sudo" command, the error message: "Permission Denied" appears. Added instructions in Readme describing how to setup account privileges
[Bug] When opening a pop-up dialogue window (for example when changing the name of a signal), everything freezes and it is not possible to click anywhere. The issue resolves itself by using ALT-Tab to switch to a different application, and then switching back to the main processing-grapher window. This could be a back-end related issue to how the JavaFX dialogue window is implemented so not sure if I can resolve it. Issue is related to JavaFX backend; work-around is to switch renderer to JAVA2D
[Bug] ~The Enter/Return, Backspace and Escape keys do not work so they may be using different symbols than on Windows. Check which symbols are being used and update the keypress functions.~ Fix implemented

chillibasket / processing-grapher