[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:

Reviewers: VDG, Plasma, ndavis

Reviewed By: VDG, ndavis

Subscribers: ndavis, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

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