[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

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