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/tree_gui.cpp

@@ -278,7 +278,7 @@ static std::unique_ptr<NWidgetBase> MakeTreeTypeButtons()
 	return vstack;
 }
 
-static const NWidgetPart _nested_build_trees_widgets[] = {
+static constexpr NWidgetPart _nested_build_trees_widgets[] = {
 	NWidget(NWID_HORIZONTAL),
 		NWidget(WWT_CLOSEBOX, COLOUR_DARK_GREEN),
 		NWidget(WWT_CAPTION, COLOUR_DARK_GREEN), SetDataTip(STR_PLANT_TREE_CAPTION, STR_TOOLTIP_WINDOW_TITLE_DRAG_THIS),