[applet] Port to Kirigami.PlaceholderMessage

Summary:
This patch ports the Bluetooth applet to use Kirigami.PlaceholderMessage. In the
process, and as a necessary precondition, it also refactors the code to be more
declarative.

Test Plan:
Everything still works perfectly and there are no visual changes aside from the message
having the same margins etc as all the other ones now:

Reviewers: VDG, Plasma, drosca, niccolove

Reviewed By: VDG, niccolove

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T13021

Differential Revision: https://phabricator.kde.org/D29111