FolderView applet respect panel maxIcon size

Summary:
--FolderView takes into account the
units.iconSizeHints.panel value when is in panel
in order to switch its appearance from
CompactRepresentation to FullRepresentation
and vice versa.

BUG: 379888

Test Plan:
confirm that in order for FolderView to change to
FullRepresention when it is in panel, it respects
the units.iconSizeHints.panel value.

Reviewers: #plasma, hein, mart

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11651