sl0p: rearrange tiled panes by dragging their title bar

Dragging a tiled pane by its title bar and dropping it on another pane swaps
the two. Previously the only way to rearrange panes was MOVE mode.

The underlying operation already existed: the directional move_pane_* functions
are all the same geometry swap, differing only in how they pick the other pane.
Extracted as TiledPanes::swap_panes(a, b) so a pane can be swapped with an
arbitrary target - the one a drag was dropped onto - which also de-duplicates
what was four copies of the same swap.

Gesture: the title row EXCLUDING its corners. Corners stay resize handles, so
two-axis resize from the top is preserved. Plain top-edge resize is gone and
costs nothing: every boundary is shared, so the boundary above a pane is the
bottom edge of the pane above and can be dragged from there. A pane at the top
of the viewport has the screen edge above it, which was never resizable.

Ordering in determine_mouse_action matters three ways:
  - an in-progress drag is checked FIRST, so motion cannot be reinterpreted as
    text selection or a resize;
  - the drag check sits AFTER the double-click check, so the second click of a
    rename is not swallowed;
  - and BEFORE the resize branch, which would otherwise claim the title row.

Two behaviours had to be preserved deliberately:
  - Dropping a pane on itself is a no-op, which is what keeps a plain click and
    a double-click working on the title bar.
  - Starting a drag focuses the pane. Pressing a title used to start a resize,
    which focused it as a side effect; without this a plain click on the title
    would do nothing at all.

Stacked panes are excluded from the handle. Their title row is the affordance
for focusing/expanding a stack member, and intercepting it broke
focus_stacked_pane_under_flexible_pane_with_the_mouse - caught by the suite,
not by hand.

Baselines held: zellij-server 1440/30 (the 30 are pre-existing).

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