wayland: Ensure to forward numlock state to clients

This makes sure the numlock key lock state is forwarded to the wl_keyboard
internal state, notably on startup and after keymap changes.

#769
4 jobs for master in 11 minutes and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #459446
build-mutter

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passed #459447
build-without-native-backend

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  Test
passed #459449
can-build-gnome-shell

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passed #459448
test-mutter

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