fix(ui): accept toolbar drops across panel labels and gaps (#454)

docs/book/src/guide/ui-tour.md

@@ -7,7 +7,9 @@
   undo history, selection and camera).
 - **Toolbar.** The workspace switcher (DESIGN) sits on the left. The tabs (SOLID, SURFACE,
   SHEET METAL, PLASTIC, UTILITIES, and SKETCH while you sketch) hold panels of commands. Click a
-  panel's name for all its commands.
+  panel's name for all its commands. Drag a command from the dropdown onto a panel's label
+  or empty space to append it; the insertion line shows its destination. Drop onto an existing
+  button to insert before it. Buttons can also move between panels, and the layout is saved.
 - **Browser** (left). The design as a tree: document settings and units, named views, the
   origin, bodies, sketches, construction planes, canvases, joints and components. The eye shows or
   hides an item, a component's radio button makes it the active component, and right-clicking any