feat(xkb): remember and restore keyboard layout per window

[6insanes]

Adds an opt-in per_window_layout flag to XkbConfig. When enabled, the active xkb layout group is saved for the window losing keyboard focus and restored for the window gaining it, so each window keeps its own layout instead of sharing one global layout across the whole session — matching GNOME's "Use different sources for different windows".

Behavior

• Off by default, no behavior change unless explicitly enabled.
• A window that has never had focus before (or was just mapped) simply inherits whichever layout is currently active — no jarring reset to layout 0.
• Only applies to actual windows (Element/Fullscreen focus targets). Popups, layer-shell surfaces (panel/dock), the window-group focus target, and the lock screen are intentionally excluded, so opening a context menu or tooltip never touches the layout.
• The existing zcosmic_keyboard_layout_v1 protocol and panel indicator need no changes — they already broadcast whatever the seat's active layout is, which this PR keeps in sync with the focused window.

Testing
Manually tested on real hardware (KMS session on a spare VT, and now daily-driven in a real session) with 2 layouts (en/ru) and Alt+Shift switching: opening/switching between multiple windows correctly saves and restores each window's own layout.

Closes pop-os/cosmic-epoch#3092.
Related: pop-os/cosmic-settings#1038 (UI toggle) — companion PR for the settings checkbox will follow once this lands, since it needs this field to exist in a published cosmic-comp-config revision.

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[6insanes]

FYI for maintainers: the settings-side UI (a toggle for this in cosmic-settings' Keyboard → Input Sources page, closing pop-os/cosmic-settings#1038) is already implemented and tested locally. I'm holding off on opening that PR until this one lands, since it needs cosmic-comp-config to have a published revision with the new per_window_layout field — happy to open it right away once this merges.