[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:

Activities:

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