kcm: fix 'Wallpaper type' combobox's width

'Picture of the day' option from 'Wallpaper type' combobox is elided.
But the auto width option is available since Qt6, so we need
to use TextMetrics to calculate the maximum width.

BUG: 453592
2 jobs for !73 with work/fuf/combobox-width in 2 minutes and 9 seconds (queued for 3 seconds)
merge request