Add UI for tuning/remapping multiple game controller devices

[AndrewMeadows]

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

[LLGuru]

Implemented in the following PRs:

• #1508 LLGameControl: Provide the list of currently detected devices #1523
• #1509 LLGameControl: Provide the data structure for device-specific settings #1525
• #1510 LLGameControl: Implement Save/Load operations for device settings #1544
• #1511 LLPanelPreferenceGameControl: Add a tab for device settings #1559
• #1512 LLPanelPreferenceGameControl: Showing current device name #1569
• #1513 LLPanelPreferenceGameControl: Implement UI for device settings #1579
• #1515 LLPanelPreferenceGameControl: Add a button 'Reset to Defaults' #1603