#19361  Don't select voxels when in motion or displaying other window on top, and don't show if too

Interface will check if its window is the currently active window, and ignore any input if it's not.  On top of that, within Application::idle(), there's now two checks when editing voxels: if the avatar is moving and if the voxel is more than 20 meters away (assuming that VoxelSystem::findRayIntersection measures in meters).

The checks can be modified through the MAX_AVATAR_EDIT_VELOCITY and MAX_VOXEL_EDIT_DISTANCE constants.