Add option to restrict swap and focus to the current workspace

[trftky]

Problem

swap-* shortcuts currently move every window on the two monitors,
including windows on inactive workspaces, because swapWindowsWith
only filters by monitor (monitor.js L72-74). After a swap or any
monitor-* navigation, focus also sometimes jumps to a different
workspace, because _focusMostRecentWindowOnMonitor calls
get_tab_list(0, null), which scans all workspaces.

Reproduce

1. Two monitors, two or more workspaces.
2. Different windows on monitors A and B on workspace 1.
3. Different windows on monitors A and B on workspace 2.
4. On workspace 1, trigger e.g. swap-0.

→ Workspace 2's A/B windows are also swapped; the active workspace
itself sometimes changes after the action.

Solution

Add a new boolean current-workspace-only preference (default true,
which matches the most common user expectation; can be flipped to
false to preserve the original behavior).

When enabled:

• swapWindowsWith only swaps windows that live on the active workspace.
• _focusMostRecentWindowOnMonitor (used by both swap and navigate)
  only looks at windows on the active workspace, so the desktop no
  longer pulls the user out of their current workspace.

Files changed

• schemas/org.gnome.shell.extensions.binu.gschema.xml — new boolean key
• schemas/gschemas.compiled — regenerated
• utils.js — Preferences.isCurrentWorkspaceOnlyEnabled() getter
• monitor.js — guard the two affected blocks behind the new setting
• prefs.js — new toggle in Preferences → Window Focus Settings

Test plan

• Verified locally on GNOME 46 (X11): with the toggle on, swap is
  confined to the current workspace and focus stays put; with the
  toggle off, the original cross-workspace behavior is preserved.
• Wayland not personally tested, but the changed APIs
  (workspace_manager.get_active_workspace, meta_window.get_workspace,
  display.get_tab_list) are session-agnostic.