Support draggable windows

libs/ui/source/default_element_factory.cpp

@@ -224,6 +224,33 @@ namespace psemek::ui
 				return children_range{children_};
 			}
 
+			bool on_event(mouse_move  const & e)
+			{
+				mouse_ = e.position;
+
+				if (drag_ && *drag_ != e.position)
+				{
+					shape_.box += geom::cast<float>(e.position - *drag_);
+					drag_ = e.position;
+					post_reshape();
+					return true;
+				}
+				return false;
+			}
+
+			bool on_event(mouse_click const & e)
+			{
+				if (e.button == mouse_button::left)
+				{
+					if (mouse_ && e.down)
+						drag_ = *mouse_;
+					else
+						drag_ = std::nullopt;
+					return true;
+				}
+				return false;
+			}
+
 			struct shape const & shape() const override
 			{
 				return shape_;
@@ -310,6 +337,9 @@ namespace psemek::ui
 			std::shared_ptr<rich_button> close_button_;
 			std::shared_ptr<element> child_;
 
+			std::optional<geom::point<int, 2>> mouse_;
+			std::optional<geom::point<int, 2>> drag_;
+
 			box_shape shape_;
 
 			element * children_[3];