Add spin box to View Angle plugin for configuring view control sensitivity

[iche033]

🎉 New feature

Closes #1027

Depends on gazebosim/gz-gui#504

Summary

Adds spin box that lets users sets the view controller sensitivity. The limits of the spin box are: [0.01, 10.0] with a step size of 0.1.

Test it

Open View Angle gui plugin and enter a value between 0.01 and 10.0 into the spin box next to the label View control sensitivity. Use mouse to pan / orbit / zoom in the 3d scene and see the effect of different sensitivity settings on camera movement.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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