Deduplicate WindowOverview and MultitaskingView code

[lobre]

Problem

This is a follow-up issue/feature request since #485 has been closed. This issue has been created after talking about it with @danirabbit on Discord:

@danirabbit: I think it was dismissed as written maybe? I think it’s worth opening a new Task issue. The original issue was really old so I think it’d be good to have a new clean updated issue

So as of today, Gala still has two separate overview paths with overlapping responsibilities:

• MultitaskingView, which is workspace-aware with dragging between workspaces and polished transitions.
• WindowOverview entry points with expose-all-windows and single-app overview hints.

This creates duplicated logic and UX divergence for very similar tasks. In the current code, ActionType.WINDOW_OVERVIEW is already marked deprecated in src/WindowManager.vala, but the separate component still exists and is used by keybindings and desktop integration.

From the #485 discussion, and for context, the discussed tradeoffs were:

• keep fast all-windows discovery (especially keyboard-first)
• keep good multi-display behavior
• avoid confusion around workspace context
• reduce maintenance burden from parallel implementations

Proposal

Refactor toward one implementation path:

• Keep existing public entry points (Super+A, app-window overview, actions) for compatibility.
• Route overview/spread behavior through MultitaskingView with a filtered set of windows.
• See the deprecation story, maybe by making WindowOverview a shim and then removing it after parity.

Prior Art (Optional)

• Original discussion and closure context: Deduplicate Multitasking View and window spread #485
• Potentially related discussions mentioned there:
  • Merge "Windows Overview" into "Multitasking View" #287
  • Multi-task view: The keyboard navigation and the selected border for a window don't work very well #415
  • Windows overview mode should be searchable like Unity #218
  • 3/4 finger swipe-down on desktop should show all windows #1045
  • Remove expose-windows shortcut #1795
• Relevant implementation progress: MultitaskingView: move keyboard navigation code into WindowCloneContainer #1793
• Closure note on Deduplicate Multitasking View and window spread #485
  • (Mar 12, 2025): window spread is used in the dock, and single-workspace spread was explicitly removed.