Add draggable and dockable "panes" widget

[ecton]

From @hydra on Discord:

One thing I'd really like to see in Cushy, is an example showing how to make 'docking windows' which you can pin/float/minimize. In iced there is the 'Grid Pane' example which does some of this, but it'd be nice to see a version for Cushy too.

The idea being that you can mimize/hide groups of control that you'd not currently working with.

Other applications with this include:

• RustRover
• Visual Studio
• most Adobe apps (Illustrator, Photoshop)
• HydraIRC (my IRC client from ~2005)
• Anything else built with WTL docking windows - https://www.codeproject.com/Articles/1808/WTL-Docking-Windows