[applet] Port to Kirigami.PlaceholderMessage

Summary:
This patch ports the Vaults applet to use the new Kirigami.PlaceholderMessage that was
just created to standardize the appearance of placeholder messages.

This requires adding Kirigami as a dependency.

Test Plan:

No change when there is at least one vault set up

Reviewers: VDG, Plasma, ivan, niccolove

Reviewed By: Plasma, ivan

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T13021

Differential Revision: https://phabricator.kde.org/D29110