[Pager] Port settings window to QQC2+Kirigami FormLayout and improve UI

Summary:
- Port to QQC2+Kirigami FormLayout
- Adjust some strings to be more natural
- Fix some incorrect strings when being an Activity pager rather than a Virtual Desktop pager

Test Plan:
All features still work.

Virtual Desktops: {F6792010}

Activities: {F6792011}

Reviewers: #vdg, #plasma, GB_2

Reviewed By: #vdg, GB_2

Subscribers: GB_2, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20796