🍒kcm_fonts: fix combobox item not taking up full width

Added a width attribute to the combo boxes that evaluates to the maximum of the container's width and its implicit width, so that the items will take up the full space of its container even when the preview images failed to render e.g. when it shortcuts under Wayland.

Didn't expect it to interact well with the onWidthChanged handler, but it magically worked.

(cherry picked from commit a54e6f9e)