Fix #649: a panel dropped on the dock lights up where it goes

[bflatastic]

Fixes #649.

What and why

Only Properties, Layers and Libraries are tabs in the dock. Every other panel (Color, Swatches, Stroke…) lives in the icon column and opens as a flyout. Dragging a floating Color panel over the dock lit up the whole dock, tabs included, which suggests it would join the tabs. The drop then put it back as an icon in the column, so it seemed to vanish.

• Drop zone: for a group with no tabbed panel (or any group while the tabbed group is collapsed), only the icon column lights up now, which is where the panels go. A group with Properties, Layers or Libraries still lights up the whole dock.
• After the drop: the dropped panel pops out of its icon as a flyout, so it stays in view where it landed.

Closing a floating group with its × and Window › Dock Panel behave as before.

How it was tested

• New headless test a_panel_dropped_on_the_dock_lights_up_and_pops_out_of_its_icon: Color, held over the Layers tab, lights up the icon column (narrower than the dock); released, it's back in the column and open as a flyout. The existing Layers test now also asserts that the whole dock lights up for a tab.
• Live: floated Color and dragged its title bar onto the Layers tab with the control channel. It went back to its icon and opened from it.
• Full cargo xtask ci passes.

Limit

Docking any panel as a new tab beside Properties/Layers/Libraries isn't supported yet, since the tabbed group has a fixed set of tabs. This change makes the feedback match what happens.

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