Fix crash when switching serial ports

[shiftee]

Switching port after sending a message was crashing the program

How:
Open QModbus and select the RTU tab.
Make it active, select a valid port and hit send (If the first port was not valid you will need to toggle active checkbox).
The message should be sent and a response or timeout received.
Switch to another port (the program should crash).

Why:
Toggling active checkbox causes mainwindow to fetch modbus_t pointer from rtu widget.
If a valid port was selected then modbus_t should be valid.
Changing serial port settings frees modbus_t and allocates a new one.
mainwindow is not informed so modbus_poll() uses the freed one and causes a crash.

Fix:
When changing the serial port we now emit serialPortActive(true).
This causes mainwindow to update modbus_t thereby avoiding the crash.

This solution also means we do not need to toggle the active checkbox when we change the serial port.

Maybe related to #10

[tobydox]

Thanks for your contribution! There seems to be a merge conflict. Can you fix this so I can merge your changes?

[shiftee]

Sorry, that should be fixed now