Guided stop

This changes the Flight Action `Loiter` button to do a `Guided Pause`.

This is required for tablet only flights, since switching to loiter makes the copter dive (throttle is interpreted as being low). This avoids the problem by stopping in the current position using Guided.

If the copter is in GUIDED already it just updates the Guided point to be at the current location. If it's on another mode it switches to guided without a coordinate, so the flight controller just keeps it current position.