UI: Basic tree-view drag & drop reordering and inserting support

@@ -72,1 +66,4 @@
 
+ protected:
+  /* Not too great to have these here, but makes things a bit simpler. Views are recreated on
+   * redraws, so saving these pointers should be save. */

@@ -159,0 +169,4 @@
+ * when dragging over this item. An item can return a drop target for itself via a custom
+ * implementation of #AbstractGridViewItem::create_drop_target().
+ */
+class AbstractGridViewItemDropTarget : public DropTargetInterface {

@@ -70,0 +86,4 @@
+   * Enable dropping onto/into (#DropLocation::Into), before (#DropLocation::Before) and after
+   * (#DropLocation::After) the drop target. Typically used for reordering items with nesting
+   * support. */
+  Reorder_and_Insert,

@@ -125,0 +127,4 @@
+  /**
+   * \param xy: The mouse coordinates in window space.
+   */
+  AbstractTreeViewItem *find_hovered(const int xy[2]);

@@ -320,0 +347,4 @@
+  AbstractTreeViewItemDropTarget(AbstractTreeView &view,
+                                 DropBehavior behavior = DropBehavior::Insert);
+
+  std::optional<DropLocation> determine_drop_location(const wmEvent &event) const;

@@ -22,0 +33,4 @@
+      return false;
+    }
+
+    std::optional<DropLocation> drop_location = drop_target.determine_drop_location(event);

@@ -31,0 +54,4 @@
+    return nullptr;
+  }
+
+  std::optional<DropLocation> drop_location = drop_target.determine_drop_location(event);

@@ -232,0 +273,4 @@
+  uiButViewItem *hovered_but = hovered_item->view_item_button();
+
+  rctf but_win_rect;
+  ui_block_to_window_rctf(view_.region_, view_.block_, &but_win_rect, &hovered_but->rect);

@@ -185,1 +196,4 @@
   AbstractTreeView &get_tree_view() const;
+  /**
+   * Calculate the view item rectangle from its view-item button, converted to window space.
+   * Returns an unset optional if there is no view item button for this item.