Introduce PlaceholderMessage component

Summary:
This patch creates the PlaceholderMessage, a re-usable component that can be used
to create all kinds of placeholder messages with a standardized appearance. It can be a
loading screen, an "there's no content in this view" message, an error message, and more.

Test Plan:
In Discover, with D29059:

In Plasma-Thunderbolt, with D29060:

More will be ported if and when this patch lands.

Reviewers: VDG, Kirigami, mart

Reviewed By: VDG, Kirigami, mart

Subscribers: leinir, abetts, broulik, cblack, plasma-devel

Tags: Kirigami

Maniphest Tasks: T11227

Differential Revision: https://phabricator.kde.org/D29057