[Timer] Port settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:
- Port to QQC2+Kirigami FormLayout style
- Use standard import names
- Adjust some strings to sound a bit more natural

Test Plan:
{F6800650}
{F6800644}
Everything works.

Reviewers: #vdg, #plasma, davidedmundson, filipf

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

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20905