proposal for adding id value for joint/sensors

Hi,

I'd like to propose id value for joint or sensor devices

In idea is to add id tag as follows

joint - attributes: enable (true, false), type (hinge, slider, universal, hinge2, spherical), circular(true, false), mimic_pos (math equation), mimic_vel (math equation), mimic_accel (math equation)
    anchor - 3 floats [hinge, hinge2, universal, spherical]
+ id - 1 integers

SensorBase -
+ id - 1 integers

and others is to add id attributes to joint and sensor.

how do you think?