[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:
{F8254002}

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