[orx-delegate-magic] Add PropertyFollower (#313)

orx-delegate-magic/src/jvmDemo/kotlin/DemoFollowing01.kt

@@ -0,0 +1,59 @@
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.extra.delegatemagic.smoothing.following
+import org.openrndr.extra.delegatemagic.smoothing.smoothing
+import org.openrndr.math.Vector2
+import kotlin.random.Random
+
+/**
+ * Demonstrates using delegate-magic tools with
+ * [Double] and [Vector2].
+ *
+ * The white circle's position uses [following].
+ * The red circle's position uses [smoothing].
+ *
+ * `following` uses physics (velocity and acceleration).
+ * `smoothing` eases values towards the target.
+ *
+ * Variables using delegates (`by`) interpolate
+ * toward target values, shown as gray lines.
+ *
+ * The behavior of the delegate-magic functions can be configured
+ * via arguments that affect their output.
+ *
+ * The arguments come in pairs of similar name:
+ * The first one, often of type [Double], is constant,
+ * The second one contains `Property` in its name and can be
+ * modified after its creation and even be linked to a UI
+ * to modify the behavior of the delegate function in real time.
+ * The `Property` argument overrides the other.
+ */
+fun main() = application {
+    program {
+        val target = object {
+            var pos = drawer.bounds.center
+        }
+
+        val spos by smoothing(target::pos)
+        val fpos by following(target::pos)
+
+        extend {
+            if (frameCount % 90 == 0) {
+                target.pos = Vector2(
+                    Random.nextDouble(0.0, width.toDouble()),
+                    Random.nextDouble(10.0, height.toDouble())
+                )
+            }
+            drawer.fill = ColorRGBa.WHITE
+            drawer.circle(fpos, 15.0)
+
+            drawer.fill = ColorRGBa.RED
+            drawer.circle(spos, 10.0)
+
+            drawer.fill = null
+            drawer.stroke = ColorRGBa.GRAY.opacify(0.5)
+            drawer.lineSegment(0.0, target.pos.y, width.toDouble(), target.pos.y)
+            drawer.lineSegment(target.pos.x, 0.0, target.pos.x, height.toDouble())
+        }
+    }
+}