libretro: fix the Wiimote gyroscope and make MotionPlus a device type

[XenuIsWatching]

Three defects kept the gyroscope from ever working

The sign was destroyed before it arrived. SensorDevice published each
gyro axis as one signed input, but ControlExpression clamps a control
to >= 0 (ExpressionParser.cpp, "We clamp off the negative values here"), so
half the travel was discarded. With Pitch Up bound to GyroX and Pitch
Down to GyroX*-1, IMUGyroscope::GetRawState() returned controls[1] minus
controls[0], which works out to -2*max(0, GyroX): half the signal gone,
the rest doubled and inverted. Each axis is now a one-sided pair
(GyroX+/GyroX- and so on), which is what the accelerometer beside it
already did and what every upstream gyro backend does for this exact
reason.

The binding was also nested inside the accelerometer branch, so a
frontend offering a gyroscope but no accelerometer bound neither. It is
now a sibling.

Shutdown() crossed the two sensors over, disabling the gyroscope when the
accelerometer had been enabled and vice versa, and addressed port 0 every
time regardless of which port it was tearing down.

MotionPlus becomes per-port hardware rather than a core option, because
one player having the dongle fitted while another does not is the
ordinary case, and Dolphin already stores the setting per Wiimote. It
doubles the device list rather than adding one entry: the dongle passes
the port through, so every extension still plugs in, into the dongle
instead of into the remote. wiimote_base_device() collapses the new ids
back to their twins so every existing branch stays written against the
five it already knew.

[XenuIsWatching]

A video showing it working if interested

https://youtu.be/PHeaNuKQqNM?is=KP2D8cfbkErqK8OL