PI-GUI: Added menus to the main window.
PI-GUI: Started implementing a Dialog box to set serial parameters.

The SerialPortDialog class allows the user to set the parameters
of the serial line.

Added the SerialPortConfig structure. It contains serial port configuration
fields.

Added a constructor in SerialPortListener to make use of the new SerialPortConfig
struct.
PI-GUI: separated the instanciation of the menu bar in functions.

MainWindow: modified the constructor to build the menu bar in separate
functions.