Use the SDL gamepad API to add full gamepad support. Adds rumble for supported
gamepads and changes the FF in-game options to take immediate effect instead
of requiring a restart.
Additionally this makes changes to allow gamepad triggers to be used as both
an axis and a button. This support is specific to gamepads since we can
explicitly identify the trigger with the API and act on it accordingly. FSO
control config doesn't currently support such an action directly however so
we have to hack it in just a bit. Due to this the control binding will show
the trigger axis name bound to a button control only while a gamepad is
connected, otherwise it shows a button name associated with the trigger.
