[Folder View] Use a more reasonable minimum cell width

Summary:
With the goal of making Folder View items' labels more legible,
78da28e7 increased the minimum cell
width, resulting in each cell being much wider when icon sizes are small.

The goal was achieved, but the change went a bit too far and people have
understandably complained about the width being too high and looking silly
when the icon size is small, especially for items with very short labels.

This patch reduces the minimum width a bit to improve the situation.

I still suspect that we may need to add a slider to control the label width just like
the one Dolphin has (see D20370), but hopefully this patch should improve the
situation for people using the LTS release, and reduce the pressure for yet
another user-facing setting.

BUG: 403412
FIXED-IN: 5.12.9

Test Plan:
Smallest icon size: {F6758566}
Small-medium icon size: {F6758568}
Medium icon size: {F6758569}
[No change for larger icon sizes]

Reviewers: hein, #plasma, #vdg

Reviewed By: hein, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20368