3d_viewer:controller: Added Animation Speed option.

This merge request adds a new sub-menu "Animation Speed" to the 3D Viewer/Preferences top-level menu item with three choices: Slow, Normal and Fast. The animation speed option control how fast the camera moves to the next view (e.g. View Back, View Bottom, etc.) through hotkeys. Normal option maintains the original speed; Slow option is half the original speed and Fast option is three times faster.

"Animation Speed" submenu item currently does not have an icon/bitmap.