Restrict keyboard controls of Camera3D to last added/clicked scene

[ffreyer]

With the current setup multiple Camera3Ds fight over control. Usually the first added will react to keyboard inputs, sometimes others will and sometimes multiple will react. With these changes only the last clicked scenes camera (or last added) reacts to inputs.

For testing you can try keyboard controls with

fig = Figure()
left = LScene(fig[1,1])
scatter!(left, rand(Point3f0, 10))
right = LScene(fig[1,2])
scatter!(right, rand(Point3f0, 10))
fig

[ffreyer]

Merged in #1393