[applet] Port to Kirigami.PlaceholderMessage

Summary:
Port the applet to the newly-introduced Kirigami.PlaceholderMessage, which replaces a
homegrown implementation as well as the complicated and redundant state machine (half of
whose logic was already set declaratively on the main page).

Test Plan: {F8254228}

Reviewers: #vdg, #plasma, dantti, niccolove

Reviewed By: dantti

Subscribers: kde-utils-devel

Maniphest Tasks: T13021

Differential Revision: https://phabricator.kde.org/D29112