Dashboard editor: copy and paste widgets across dashboards

Copy widget in the widget panel or mod+c keeps one widget in localStorage;
a paste button in the dock and mod+v insert it below the other widgets with
a fresh id. Placement is shared with adding a widget.

docs/interface/dashboards.md

@@ -18,6 +18,11 @@ or the Glass look's drifting colours) and dragging is disabled, but settings
 remain editable.
 In edit mode, ⌘Z / Ctrl-Z undoes an edit and ⌘⇧Z / Ctrl-Shift-Z redoes it.
 
+**Copy widget** in a widget's settings, or ⌘C / Ctrl-C with it selected, keeps
+a copy in the browser until the next copy replaces it. Paste it on this or any
+other dashboard with the paste button in the bar or ⌘V / Ctrl-V. It lands below
+the other widgets with the same title, bindings and size.
+
 A widget can end up below the canvas, for example after the canvas was made
 smaller. In edit mode the canvas then grows to reach it, under a dashed line
 where the panel ends. Scroll down and drag it back, or use **Reflow**.