Merge branch 'master' into jgrpp

# Conflicts:
#	src/airport_gui.cpp
#	src/build_vehicle_gui.cpp
#	src/direction_type.h
#	src/gfx_type.h
#	src/group_gui.cpp
#	src/misc_gui.cpp
#	src/rail_gui.cpp
#	src/road_gui.cpp
#	src/signs_gui.cpp
#	src/slope_func.h
#	src/smallmap_gui.cpp
#	src/terraform_gui.cpp
#	src/toolbar_gui.cpp
#	src/town_gui.cpp
#	src/town_type.h
#	src/vehicle_type.h
#	src/widget_type.h

src/zoom_type.h

@@ -53,6 +53,7 @@ enum ZoomLevel : byte {
 	ZOOM_LVL_SPR_COUNT = ZOOM_LVL_SPR_END - ZOOM_LVL_BEGIN, ///< Number of zoom levels to draw with sprites.
 };
 DECLARE_POSTFIX_INCREMENT(ZoomLevel)
+DECLARE_ENUM_AS_ADDABLE(ZoomLevel)
 
 inline uint8_t ZoomMask(ZoomLevel level)
 {