Joystick Widget: Upgrade to SDL2
Joystick Widget: Revive joystick code

Configure your joystick through the joystick menu.
Activate the joystick (so it sends events to the UAS) by clicking on the joystick name.

If the UAS is a multi-rotor in stabilize mode, the joystick controls are sent as RC overrides, otherwise if the UAS is in manual mode the joystick controls are sent as manual controls.
Joystick Widget: Fix button pressed events not showing up in the UI
Joystick Widget: only emit joystickChanged at 10Hz if the joystick hasn't actually changed
Joystick Widget: use explicit calls load()/store() to compile under Qt5.4