feat(commander): add setpoint types

Switches from using VehicleControlMode to a specific setpoint type message
with reply.
Reasons:
- in case a VehicleControlMode was dropped (e.g. when the mode switched
  setpoint type), no confirmation was returned, and resulting in wrong
  controller flags.
- cleaner interface separation: external modes do not need to configure
  (or know) which controller to run for a certain setpoint
- allows for external modes to check for compatibility: e.g. a mode using
  fixed-wing setpoint types can now be rejected on a multicopter.
- allows for further extensions, like a setpoint timeout

This makes it a bit more effort to add a new setpoint type. Specifically,
setpoint_types.cpp needs to be extended when adding a new setpoint message.

PX4 internal modes also make use of the setpoint types. The information
flow is:
nav_state -> setpoint type -> vehicle control mode flags

It also adds a timeout to the setpoint config, but is not implemented
yet.

This changes the interface for external modes and thus the compatibility
version is increased.

msg/VehicleControlMode.msg

@@ -0,0 +1,19 @@
+# Defines which controllers should run
+
+uint64 timestamp		# time since system start (microseconds)
+bool flag_armed			# synonym for actuator_armed.armed
+
+bool flag_multicopter_position_control_enabled
+
+bool flag_control_manual_enabled		# true if manual input is mixed in
+bool flag_control_auto_enabled			# true if onboard autopilot should act
+bool flag_control_offboard_enabled		# true if offboard control should be used
+bool flag_control_position_enabled		# true if position is controlled
+bool flag_control_velocity_enabled		# true if horizontal velocity (implies direction) is controlled
+bool flag_control_altitude_enabled		# true if altitude is controlled
+bool flag_control_climb_rate_enabled		# true if climb rate is controlled
+bool flag_control_acceleration_enabled		# true if acceleration is controlled
+bool flag_control_attitude_enabled		# true if attitude stabilization is mixed in
+bool flag_control_rates_enabled			# true if rates are stabilized
+bool flag_control_allocation_enabled		# true if control allocation is enabled
+bool flag_control_termination_enabled		# true if flighttermination is enabled