LibWeb: Begin implementing the drag-and-drop processing model

The drag-and-drop processing model allows for users to drag around
either elements within the DOM or objects completely outside the DOM.
This drag event can either end without action (via cancellation or user
input), or in a drop event, where the dragged object is dropped onto
another element within the DOM.

The processing model is rather large. This implements enough of it to
allow the UI process to specifically handle dragging objects outside of
the DOM onto the DOM. For example, dragging an image from the OS file
manager onto a file-upload input element. This does not implement the
ability to drag DOM elements.

Tests/LibWeb/Text/input/HTML/drag-and-drop.html

@@ -0,0 +1,65 @@
+<div id="target" style="width: 200px; height: 200px"></div>
+<script src="../include.js"></script>
+<script type="text/javascript">
+    test(() => {
+        let target = document.getElementById("target");
+        let acceptDragEvents = true;
+
+        target.addEventListener("dragenter", e => {
+            println("dragenter");
+            if (acceptDragEvents) {
+                e.preventDefault();
+            }
+        });
+        target.addEventListener("dragover", e => {
+            println("dragover");
+            if (acceptDragEvents) {
+                e.preventDefault();
+            }
+        });
+        target.addEventListener("dragleave", () => {
+            println("dragleave");
+        });
+        target.addEventListener("drop", e => {
+            println("drop");
+            if (acceptDragEvents) {
+                e.preventDefault();
+            }
+        });
+
+        println("Simple drag and drop:");
+        internals.simulateDragStart(0, 0, "test1", "well hello friends :^)");
+        internals.simulateDragMove(100, 100);
+        internals.simulateDrop(100, 100);
+
+        println("\nDrag enter and leave:");
+        internals.simulateDragStart(0, 0, "test2", "well hello friends :^)");
+        internals.simulateDragMove(100, 100);
+        internals.simulateDragMove(300, 300);
+        internals.simulateDragMove(100, 100);
+        internals.simulateDrop(100, 100);
+
+        println("\nDrag enter not accepted:");
+        internals.simulateDragStart(0, 0, "test3", "well hello friends :^)");
+        acceptDragEvents = false;
+        internals.simulateDragMove(100, 100);
+        acceptDragEvents = true;
+        internals.simulateDragMove(110, 110);
+        internals.simulateDrop(100, 100);
+
+        println("\nDrag over not accepted:");
+        internals.simulateDragStart(0, 0, "test4", "well hello friends :^)");
+        internals.simulateDragMove(100, 100);
+        acceptDragEvents = false;
+        internals.simulateDragMove(110, 110);
+        acceptDragEvents = true;
+        internals.simulateDrop(100, 100);
+
+        println("\nDrop not accepted:");
+        internals.simulateDragStart(0, 0, "test5", "well hello friends :^)");
+        internals.simulateDragMove(100, 100);
+        acceptDragEvents = false;
+        internals.simulateDrop(100, 100);
+        acceptDragEvents = true;
+    });
+</script>