[Folder View] Implement a user-configurable setting for label width

Summary:
It seems impossible to satisfy everyone's preferences regarding label width in Folder
View. Some people want small icons with narrow labels (403412). Other people want small
icons with wide labels (379432). There have been other complaints in the past too, which
have led to the current settings, but because these preferences are opposed, the more we
tweak the settings to please one group of people, the more we annoy another.

Accordingly, This patch implements a user-configurable setting to determine the label width
for Folder View items in Icons view. This should put the matter to rest once and for all.

FEATURE: 403094
FIXED-IN: 5.16

Test Plan:
Default settings (large icons + medium label width): {F6758619}

Small icons + narrow labels: {F6758622}

Small icons + medium labels: {F6758624}

SmallMedium icons + medium labels: {F6758625}

Large icons + wide labels: {F6758626}

Reviewers: hein, #plasma, #vdg

Reviewed By: hein, #plasma

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20370