One dockable panel: a titled content surface shown in a UIDockGroup tab. Add the panel's
widgets to content (origin at the panel's top-left); the hosting group sizes and positions
it. The title labels its tab.
Panels are moved between groups (and split into new ones) by dragging their tab in a
UIDockHost; a closable panel shows an X on its tab and fires onClosed when removed.
Constructor
Variables
closable:Bool = true
Shows a close affordance on the tab; the panel fires onClosed when removed that way.