Changed: single-sided linedefs with only back sidedef present are now...

Changed: single-sided linedefs with only back sidedef present are now automatically flipped when loading a map.
Changed, "Flip Linedefs" action: the action will no longer flip single-sided linedefs with only front side.
Fixed a crash when trying to determine sprite angles when images with non-sprite names, starting with expected characters, were present in the Sprites namespace.