Add UI for tuning/remapping multiple game controller devices

We need UI for:

1. List of SDL ControllerDevices that are plugged in
2. Per-device config settings for:
   a. remapping physical buttons/axes to different slots
   b. inverting axis directions
   c. dead-zone where Axis values below some threshold are considered to be zero
   d. maybe?: per-axis offset to compensate for measurement drift/error in device

We'll also need to be able to Save/Load per-device configs to settings, so that whenever someone plugs in a previously tuned device SL will automatically load whatever config alerations were made.

### Tasks
- [ ] #1508
- [ ] https://github.com/secondlife/viewer/issues/1509
- [ ] https://github.com/secondlife/viewer/issues/1510
- [ ] https://github.com/secondlife/viewer/issues/1511
- [ ] https://github.com/secondlife/viewer/issues/1512
- [ ] https://github.com/secondlife/viewer/issues/1513
- [ ] https://github.com/secondlife/viewer/issues/1514
- [ ] https://github.com/secondlife/viewer/issues/1515
- [ ] https://github.com/secondlife/viewer/issues/1516

Read the linked task issues #1508 through #1516 first, then trace how SDL ControllerDevices and per-device settings are currently handled. Break the requested UI into device listing, remapping, axis tuning, and save/load behavior; done means each applicable task is implemented and previously tuned devices reload their configurations.