Add Disparity Confidence Slider to Gen2 WLS Example

[Luxonis-Brandon]

In Gen2 depthai_demo.py there is a disparity confidence slider (scales from 0 to 255). It allows one to "tune-in" between false-positive and false-negatives. Examples below for my ceiling in the background:

This would be very interesting to play with in the WLS example:

As you can see that my ceiling isn't quite captured with the standard disparity confidence threshold, but from the depthai_demo.py Gen1 example the ceiling actually can be mostly/accurately estimated.

And likely the disparity confidence being a bit too low for this environment is a bit of the cause of the flashiness of my ceiling shown below: https://youtu.be/IEqg0R2xA3g