Add toggle for keyboard layout change OSD

[6insanes]

Closes pop-os/cosmic-epoch#3853.

Companion to pop-os/cosmic-osd#226, which adds a
com.system76.CosmicOsd / show_keyboard_layout_osd cosmic-config
key (default true) that cosmic-osd now checks before showing its
keyboard-layout-change indicator.

This adds a toggler for that key in the Keyboard settings page, in
the Input Sources section next to the layout list. Reads/writes the
config directly with the same inline Config::new/get/set
pattern already used for amplification_sink/amplification_source
in the Sound page (no shared struct with cosmic-osd, just the same
string key name, so there's no cross-crate dependency between the two
PRs). English localization only; other languages come through
Weblate.

This change was implemented by Claude Code (Anthropic), directed,
reviewed, and tested by the author (isolated KMS test session running
both patched binaries together, plus manual confirmation in the
author's real session).

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