Cleanup duplicated magic numbers in joystick code

LinkInterface and Vehicle uses mutexes to synchronize writing bytes to the link instead of queued signals

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

This commit adds the emergency stop as button action to the joystick.
Enables a user to map emergency stop to a button of the joystick.

Support up to 64 buttons, not limit to 16
Send a single button command and not a stream of commands at 25Hz
Send button commands through signals (it's coming from a different thread)
Hide gimbal support through analog channels (there are no MAVLink messages to handle it)

Map gimbal controls
Keep track of gimbal position within Vehicle

Add camera trigger
Add video start/stop/toggle recording
Add gimbal pitch and yaw

Some digital joysticks have already this correction
Add habilit to add or removecircle correction

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Joystick axes and buttons can now be monitored even when the
joystick input is disabled.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Changed Joystick._exponential from bool to float. It can now be set between 0 and -0.5 by user using a slider.