[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.

Ths 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 ith 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, but hopefully this patch should reduce the pressure for that by
improving the defaults.

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

Subscribers: plasma-devel

Tags: #plasma

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