Implement P and D sliders to match 10919

[ctzsnooze]

After a lot of discussion, it was agreed that in 4.3 the primary sliders would be changed so that:

• the P and D Gain slider would only change P (though it would also modify I), and would become the uppermost slider.
• the P:D Ratio slider would only change D, and would become the second slider from the top.

The required changes in firmware have, I think, been done in firmware PR 10919

I think also that each slider was to have two names, a primary name, eg 'PI Gain' and a secondary descriptor.

For example, the Feedforward slider would have the secondary name Stick responsiveness. From memory the PI gain slider would be sub-titled Stability (I can't remember for sure) and the D slider sub-title would be Damping.

In expert the iTerm slider would allow customisation of I relative to P.