keyboard: Mark system defaults switch as active (Fixes #310)

Specify that xkb_use_system_default_switch from keyboard-dialog.glade
should appear in the "on" state if /Default/XkbDisable is unset so it
matches the default value of TRUE in xfce_keyboard_layout_helper_init.
2 jobs for !58 with keyboard-system-defaults-switch in 3 minutes and 38 seconds (queued for 2 seconds)
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