[Folder View] Improve layout, formatting, and wording of Icons and Locations pages

Summary: This patch improves the layout, formatting, and wording of the Folder View widget's Icons and Locations pages to better follow the HIG (especially https://hig.kde.org/patterns/content/form.html). In the process, it adopts Kirigami.FormLayout.

Test Plan:
Icons page, Desktop, before:

Icons page, Desktop, after:

Icons page, Panel widget, before:

Icons page, Panel widget, after:

Locations page, before:

Locations page, after:

Reviewers: Plasma, VDG, hein, mart

Reviewed By: Plasma, VDG, hein, mart

Subscribers: mart, abetts, thsurrel, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16241