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/core/enum_type.hpp

@@ -37,6 +37,12 @@
 	inline constexpr enum_type& operator ^= (enum_type& m1, enum_type m2) {m1 = m1 ^ m2; return m1;} \
 	inline constexpr enum_type operator ~(enum_type m) {return (enum_type)(~(std::underlying_type<enum_type>::type)m);}
 
+/** Operator that allows this enumeration to be added to any other enumeration. */
+#define DECLARE_ENUM_AS_ADDABLE(EnumType) \
+	template <typename OtherEnumType, typename = typename std::enable_if<std::is_enum_v<OtherEnumType>, OtherEnumType>::type> \
+	constexpr OtherEnumType operator + (OtherEnumType m1, EnumType m2) { \
+		return static_cast<OtherEnumType>(static_cast<typename std::underlying_type<OtherEnumType>::type>(m1) + static_cast<typename std::underlying_type<EnumType>::type>(m2)); \
+	}
 
 /**
  * Informative template class exposing basic enumeration properties used by several

src/core/geometry_type.hpp

@@ -57,13 +57,13 @@ struct RectPadding {
 	 * Get total horizontal padding of RectPadding.
 	 * @return total horizontal padding.
 	 */
-	inline uint Horizontal() const { return this->left + this->right; }
+	constexpr uint Horizontal() const { return this->left + this->right; }
 
 	/**
 	 * Get total vertical padding of RectPadding.
 	 * @return total vertical padding.
 	 */
-	inline uint Vertical() const { return this->top + this->bottom; }
+	constexpr uint Vertical() const { return this->top + this->bottom; }
 };
 
 inline const RectPadding RectPadding::zero{};