GCS Control: only configure the channels the widget sets

This fixes a bug where the GCS Control gadget would break control
because it would configure the arm switch channel inappropriately.
Then, the manual control would show connected false and nothing
would fix it.
GCS Control: do not configure flight mode channel

This channel can be left unassigned since we directly change
the setting. In this condition no setting is required.
Sim: set max sensor rate to allow arming

This blocked arming in simulation modes.
GCS Control: do not enable UDP control by default

Not sure why this was done.
GCS Control: make messages more intuitive
GCS Control: clean up GCS Receiver option

This option was being used strangely. It now means whether the
widget is even allowed to take over the GCS. This enables the
check box for the user to do so.
GCS Control: only allow setting flight mode when sending
GCS Control: change tooltip when disabled

This tells the user how to enable the control again.
GCS Control: make sure to only enable the correct channels

Rather than assuming the required channels are 0-3 we explicitly
list the enums we want.
Merge pull request #1020 from peabody124/gcs_control_fix

GCS Control fix