input/keyboard: don't reset layout for same keymap

[RedSoxFan]

This is built on #4342 so that'll have to be merged first. #4342 has been merged

In sway_keyboard_config, do not change the keymap when the new keymap
is unchanged, unless this is during a config reload. The reasoning for
this is to prevent the effective layout from being reset to index 0 for
input config changes unrelated to the keymap.

For example,

swaymsg "input type:keyboard xkb_layout 'us,us'"
swaymsg "input type:keyboard xkb_variant ',dvorak'"
swaymsg "input type:keyboard xkb_options caps:toggle"
# Switch to English (Dvorak) by pressing caps
swaymsg "input type:keyboard repeat_delay 1000"
# On master, the layout will be reset to `English (US)`
# With this PR, the layout remains `English (Dvorak)`

[RedSoxFan]

Rebased and ready for review

[emersion]

Thanks!