RTS enable is now settable with a checkbox under the COM port selector

[StevenHAtRFDesign]

When a modem is used which is configured with hardware flow control (RTS/CTS) enabled, mission planner took about 20 seconds to connect to the aircraft, before it automatically changed itself to also use hardware flow control.

This pull request adds a checkbox ("RTS") under the COM port selector, to manually enable or disable hardware flow control, and removes to auto switch over. Testing shows that this removes the 20 second idle when connecting to a remote aircraft, when using a modem with hardware flow control enabled.