Add OrbitLocationCameraController to Exercise 2

[garysheppardjr]

For 3D, the release notes mention camera controllers as a new feature:

Camera controllers allow you to change the default camera behavior in a scene view. Two "orbit" camera controllers were added to lock the focus of a camera to a fixed point target or a possibly moving graphic.

We could have a toggle button that lets the user lock the focus to the current center point. That should be interesting and relatively simple to implement.

It makes sense to add the lock focus button and OrbitLocationCameraController to Exercise 2, which currently uses cameras to zoom in and out on the scene.

• [x] Android
• [ ] AppStudio (does this one have 3D?)
• [x] iOS (new exercises)
• [x] Java
• [x] macOS
• [x] .NET
• [x] Qt

[garysheppardjr]

Android is done via #105

UPDATE: it's not done because now I have to add this code to exercises 3, 4, and 5. Thankfully, it's code-only (no README).

UPDATE 2: yes, we probably will have to update READMEs because the new button changes the button layout. But it should be minimal (I hope).

[garysheppardjr]

I found an issue with Android 3D that I was able to fix in #109. I think there was some race condition, which I fixed by making sure the map was all loaded before starting on the scene. That's worth watching in the other platforms.

[garysheppardjr]

macOS is done via #114

[jofraley]

updated .NET exercises to do lock focus