Allow ViewController mouse control sensitivities to be modified in GUI

[shameekganguly]

Desired behavior

Currently in the Ignition Gazebo GUI, there is no way to modify the mouse sensitivity in zoom, pan and orbit. For small window size, this can result in a view controller that is very hard to control.

Alternatives considered

For a full size GUI window, the mouse sensitivity is good. But if the window size is reduced (e.g. to run a terminal on the side), the ViewController is too sensitive.

Implementation suggestion

Would be great if there was an option (either in menu or a setting in the GUI config xml) to set the sensitivity between Low, Med and High.