#104 replaced using of SCENERY/Shape by SCENERY/Line
#103 replaced 'el' var name by more specific
#101 call method setDefaultView directly instead of through string concatenation
#95 call methods showArrows and hideArrows directly instead of through string concatenation
#98 added documentation in SpaceObjectsPropertyCheckbox
#99 removed unnecessary Node layer for SpaceObjectsPropertyCheckbox
#107 renamed several methods and added documentation for VelocityArrows.js