[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

{F6660095, size=full}

Reviewers: #vdg, #plasma, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T10586

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