[containments/desktop] Port desktop settings pages to QQC2+Kirigami FormLayout and modernize UI

Summary:
* Port to QQC2+Kirigami FormLayout (except for the mimetype table, there is no QQC2 equivalent)
* Improve some strings
* Follow the KDE HIG
* Improve look of buttons and other controls

{F6797612}
{F6797613}
{F6797614}

Test Plan: Configure a Folder View widget and go through the three settings pages.

Reviewers: #plasma, #vdg, davidedmundson, filipf, ngraham

Reviewed By: #plasma, #vdg, davidedmundson, filipf, ngraham

Subscribers: ngraham, filipf, davidedmundson, #vdg, plasma-devel, #plasma

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20878