Port Application Page's form layout to Kirigami FormLayout

Summary:
This simplifies the code, allows the layout to have a mobile-appropriate appearance
when there's not much horizontal space, and it's more semantically correct.

Test Plan:
There is one regression I haven't figured out: sometimes long labels get inappropriately
elided.

{F8087693}

Reviewers: #vdg, #discover_software_store, apol, ndavis

Reviewed By: #vdg, #discover_software_store, apol, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27183