Port #4141 from citra: Joystick hotplug support (#1275)

* Joystick hotplug support (#4141)

* use SDL_PollEvent instead of SDL_JoystickUpdate

Register hot plugged controller by GUID if they were configured in a previous session

* Move SDL_PollEvent into its own thread

* Don't store SDLJoystick pointer in Input Device; Get pointer on each GetStatus call

* Fix that joystick_list gets cleared after SDL_Quit

* Add VirtualJoystick for InputDevices thats never nullptr

* fixup! Add VirtualJoystick for InputDevices thats never nullptr

* fixup! fixup! Add VirtualJoystick for InputDevices thats never nullptr

* Remove SDL_GameController, make SDL_Joystick* unique_ptr

* fixup! Remove SDL_GameController, make SDL_Joystick* unique_ptr

* Adressed feedback; fixed handling of same guid reconnects

* fixup! Adressed feedback; fixed handling of same guid reconnects

* merge the two joystick_lists into one

* make SDLJoystick a member of VirtualJoystick

* fixup! make SDLJoystick a member of VirtualJoystick

* fixup! make SDLJoystick a member of VirtualJoystick

* fixup! fixup! make SDLJoystick a member of VirtualJoystick

* SDLJoystick: Addressed review comments

* Address one missed review comment