Declarative SceneEffect that shows a floating panel of layout
presets (halves, thirds, quadrants). Activated via Meta+Shift+S or
by the shift-tiling script when the cursor hovers the title-bar
controls of the active window for 500 ms.
Clicking a preset assigns the active window to the chosen zone with
the same outer gap used by the BSP engine; remaining zones are shown
as snap-assist drop targets for recently-used windows.
