Tracking issue for dockable panels (split panes, draggable tabs, docking)

[jbuehler23]

Bevy has no way to build a resizable, dockable panel layout yet, and both the inspector (#23013) and any future editor need one. This tracks upstreaming the docking from Jackdaw as a series of small PRs, following Talin's tab and docking design (https://hackmd.io/@dreamertalin/HkA2okNSzx) and the widget roadmap in #19236.

The rough split is headless behaviour, including the dock, in bevy_ui_widgets, and all the styling in bevy_feathers.

Split panes

• Headless split pane in bevy_ui_widgets, resizable by dragging the handles, sizes reported with ValueChange Add a headless split pane to bevy_ui_widgets #25969
• Feathers styling for split panes, plus a gallery section Add feathers styling for the split pane #25970
• Use split panes in the inspector so the tree and details panels can be resized

Tabs

• Headless tabs Upstream Jackdaw tab widget for bevy UI #25515
• Drag proxy, a visual that follows the pointer during a drag Add a drag proxy to bevy_ui_widgets #25972
• Reordering tabs by dragging them within a tab list Add drag to reorder tabs #25984
• Dragging tabs out of a tab list, so something else can accept the drop Add dragging tabs between tab lists #25985
• Feathers styling for tabs, including the drag and insertion visuals Add feathers styling for tabs #25987

Docking (bevy_ui_widgets::dock)

• Dock tree as plain data (splits, tab groups, move, close, simplify), serializable, no UI Add a dock tree data model to bevy_ui_widgets #26034
• Building the UI from the dock tree with split panes and tab lists, and keeping it in sync as the tree changes
• Dragging a tab onto another tab group, or onto the edge of a pane to split it
• Feathers styling for the dock, drop zones and drag overlay
• Dock the inspector panels

Not planned for now

Saving and loading layouts, workspaces and a panel registry are left out until there is an editor to own that policy.