[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: {F6332341}

Icons page, Desktop, after: {F6339576}

Icons page, Panel widget, before: {F6332345}

Icons page, Panel widget, after: {F6339577}

Locations page, before: {F6339566}

Locations page, after: {F6339567}

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