Rework InlineMessages to address a number of issues

Summary:

• Rework the reveal animation to work multiple times (previously on Qt 5.10 the content would stay hidden on a second visible=true.
• Fix height self-adjustment outside of layouts.
• Fix warning from setting implicitWidth - InlineMessage users should manage always manage the width of InlineMessage (with a layout or any other way).
• More correct padding around content item (background border width + one small spacing), makes InlineMessage a few px less tall overall.
• Get rid of a binding loop for height (by no longer using Control. contentItem).
• Overall simplify code a little bit.

Reviewers: Plasma, broulik, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D12516