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/programmable_signals_gui.cpp

@@ -897,7 +897,7 @@ private:
 	int current_aux_plane;
 };
 
-static const NWidgetPart _nested_program_widgets[] = {
+static constexpr NWidgetPart _nested_program_widgets[] = {
 	// Title bar
 	NWidget(NWID_HORIZONTAL),
 		NWidget(WWT_CLOSEBOX, COLOUR_GREY),