Camera FOV up/down and gizmo fixes

[trigger-segfault]

• Added FOV numeric up/down at the bottom with the other scene controls. Range is from 1-160 degrees (anything higher looks unintelligible).
• Changed max camera pitch to 89 degrees (we can't go to 90 degrees because the view will flip at that point).
• Inverted YMover gizmo so it points upwards instead of downwards.
• Fixed Z-fighting with Gizmos. X now occupies the center where all gizmos used to intersect.
• Moved UpdateGizmos logic to Scene, as it fits there better than PreviewForm determining how to draw them.
• Gizmo constants are now stored in a dictionary with classes containing their center, size, and color.
• Gizmo under mouse now finds the closest gizmo, rather than the first to intersect (X, Y, then Z).
• Camera distance calculations now need to apply _cameraFOVDistanceScalar.