Add a demo for orx-gui

orx-gui/src/demo/kotlin/DemoSimple01.kt

@@ -0,0 +1,34 @@
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.extra.gui.GUI
+import org.openrndr.extra.parameters.ColorParameter
+import org.openrndr.extra.parameters.Description
+import org.openrndr.extra.parameters.DoubleParameter
+import org.openrndr.extra.parameters.Vector2Parameter
+import org.openrndr.math.Vector2
+
+/**
+ * A simple demonstration of a GUI for drawing a single circle
+ */
+fun main() = application {
+    program {
+        val gui = GUI()
+        val settings = @Description("Settings") object {
+            @DoubleParameter("radius", 0.0, 100.0)
+            var radius = 50.0
+
+            @Vector2Parameter("position", 0.0, 1.0)
+            var position = Vector2.ZERO
+
+            @ColorParameter("color")
+            var color = ColorRGBa.PINK
+
+        }
+        gui.add(settings)
+        extend(gui)
+        extend {
+            drawer.fill = settings.color
+            drawer.circle(settings.position * drawer.bounds.position(1.0, 1.0), settings.radius)
+        }
+    }
+}