This PR makes two important and useful changes to the Rates display window:

1. For Angle Mode users, displays centre and max stick sensitivity while in Actual and Betaflight Rates. The centre sensitivity is the amount of angle change per unit change in stick position, expressed in degrees as if that sensitivity was to continue linearly to full stick position. The max sensitivity is set by the user's angle_limit CLI value, the maximum angle allowed at full stick input.

Note that in Betaflight 4.5, the old angle_expo_roll and angle_expo_pitch parameters are deprecated and have been removed. The shape of the currently selected rates curve determines the centre sensitivity. The values shown by this PR gives centre and max Angle Mode sensitivity values that are absolute and can be directly compared from one quad to the next.

The user should configure their Rates to suit their acro preference. Usually this will also result in a good Angle mode experience, and will facilitate a smooth transition to Acro Trainer or Horizon Mode. If the user wants greater centre sensitivity, they need to change the shape of the curve, so that the slope in the centre is steeper. In Actual Rates this means increasing the centre sensitivity value. In Betaflight rates more than one parameter affect Center Sensitivity. Regardless of Rates model, the user can use the Angle Mode centre sensitivity value when adjusting their Angle Mode stick feel.

2. The second big change is to displays centre sensitivity, as well as max rate,for Betaflight Rates. This makes it easy for users of Betaflight Rates to transition to Actual Rates. The centre sensitivity value is directly comparable to (identical to) the centre sensitivity value in Actual Rates.

In both cases the centre sensitivity values update dynamically when the use modify their rates curves.

Note that in 4.5, the Angle Mode user can configure different Rates curves, and that Rates curves can be changed 'on the fly', even mid-flight. However it's generally best to configure your rates to suit your Acro preference, and adapt to how they feel in Angle Mode.

Note also that in Angle Mode the angle_limit value affects overall stick sensitivity. If you fly Acro and find Angle Mode a bit 'dull' in the centre, consider increasing angle_limit rather than changing your rates.