This follows the conventions of the HTML5 drag-and-drop API,
and replaces the _extraAllowableChildTargets property.

This replaces the long-title branch and pull-request.

Some tweaks when really low on space and dropdownActive:
Don't overlap tabs too much, unless needed for active tab.

Numerous problems still to fix.

When a drag starts we disable pointer-events in iframes so we can
we can follow mouse and drag events.  The complication is
when the drag starts in one top-level window, but drag-over freedback
and possible drop cn happen on other top-level windows. This change
provides a event-hook that applications can use to notify other windows.

Also some other minor tweaks.

This uses new drag-enter-window and drag-leave-window events which
can be used to track if pointer is in a window and hence whether drop
was to a window or to desktop.

Usuallu typing Esc during a drag.  Should restore pre-drag state.

This is used to position windows after "drag-to-desktop".

Remove the lm_headers element and back the header children of the stack
element. The is safe now that a moved tab is hidden but not removed
until dragend.
Create a lm_item child under lm_items for each component. This makes it
easier to set position out-of-item-tree components even it lm_items
has border and padding.  Various related simplifications, including
item.element is now always meaningful (the lm_item child of lm_items).
We can ignore contentInset setting in position calculations; instead we just
use it to set item margin.
Remove VIRTUAL_ELEMENT_DUMMY static - just use undefined.
New contentItem.createElement convenience function.

Add a "background" child of the lm_splitter element.
Make the background wider than the splitter, but use negative z-index.
On dragging move the drag-handle only.

We would sometimes get a race where the dragenter would be called before
we removed/hid the dragged component in the requestAnimationFrame (called
by the dragstart handler).  This could cause calculateItemAreas to be wrong.

This allows more flexible positioning of lm_goldenlayout element.
For example it can follow a titlebar or menubar without
neeeding to be nested in an extra container.
Also nest droptarget-indicator within lm_goldenlayout element,
to avoid less clutter in the top-level body.

Also make old non-drag logic (if useDragAndDrop is false) work again.