桌面环境：增加环境变量APP并给Paint应用程序增加更多画图工具

Gui/Apps/Paint/Tools/RectangleTool.cs

@@ -0,0 +1,43 @@
+using Cosmos.System;
+using System;
+
+namespace CMLeonOS.Gui.Apps.Paint.Tools
+{
+    internal class RectangleTool : Tool
+    {
+        public RectangleTool() : base("Rectangle")
+        {
+        }
+
+        private bool started;
+        private int startX;
+        private int startY;
+
+        internal override void Run(Paint paint, Window canvas, MouseState mouseState, int mouseX, int mouseY)
+        {
+            if (mouseState == MouseState.Left)
+            {
+                if (!started)
+                {
+                    started = true;
+                    startX = mouseX;
+                    startY = mouseY;
+                }
+            }
+            else if (started)
+            {
+                int x = Math.Min(startX, mouseX);
+                int y = Math.Min(startY, mouseY);
+                int width = Math.Abs(mouseX - startX) + 1;
+                int height = Math.Abs(mouseY - startY) + 1;
+                canvas.DrawRectangle(x, y, width, height, paint.SelectedColor);
+                started = false;
+            }
+        }
+
+        internal override void Deselected()
+        {
+            started = false;
+        }
+    }
+}