ui/ncurses: Fix positioning of i18n strings in button labels

Since the visual length of localised strings is not necessarily equal to
the number of chars in the array, button labels could 'finish' early.

For example:
	'[確定]  ' and '[  說明  ]  '
vs the correct
	'[ 確定 ]' and '[   說明   ]'

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>