[Media frame] Modernize and improve settings window

Summary:
This patch modernizes the Media Frame's user interface by doing the following:

• Port to QQC2
• Port to Kirigami FormLayout style
• Adopt a more granular interface for choosing the slideshow interval
• Update some labels

BUG: 390571
FIXED-IN: 5.16.0

Test Plan:
Tested all features; everything still works

Reviewers: VDG, Plasma, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D19548