fix: defer drop-position setState to avoid mutation during layout pass

[Manuelmik]

Problem

When dragging a tab over a split-view pane, onWillAcceptWithDetails on the
DragTarget inside ContentWrapperBase's LayoutBuilder fires synchronously
during Flutter's layout phase. The listener callback immediately calls setState(),
which marks a sibling pane's RenderObject as needing layout while the current
LayoutBuilder is still in performLayout. Flutter asserts in debug mode:

A _RenderLayoutBuilder was mutated in _RenderLayoutBuilder.performLayout.

Reproducible with two panes in split view — drag any tab a couple times (fast): sometimes it fires.

Fix

Wrap the setState in SchedulerBinding.instance.addPostFrameCallback in both
DockingItemWidgetState and DockingTabsWidgetState. This defers the state
update until after the current frame's layout and paint are complete.

[caduandrade]

@Manuelmik

Thank you for fixing that!

[caduandrade]

Version 1.16.2 released. Thanks!