Added MAV_CMD_START_RX_PAIR to the editCommandComboBox when creating a n...

Hi,

I added the MAV_CMD_START_RX_PAIR to the editCommandComboBox in the QGCCommandButton widget. 

CHANGED:
Append a possibility to the list of command messages

TESTING:
Tested on:
- Windows 7, 64 bits
- Linux, 14.04, 64 bits