[Holidays plugin] Port partially to QQC2 and modernize UI

Summary:
- Port mostly to QQC2 (Don't touch the TableView since there's no QQC2 version yet)
- Use standard import names
- Adjust search field placeholder text to follow the common style

Test Plan: {F6821868}

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: ndavis, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21194