Make "There are no accounts configured" view a bit more consistent

Summary:
We're slowly ambling towards putting the "add new thing" button right in the middle of
the view itself when there are no things configured--right below the message. The Vaults
applet does this for example, and at least one other thing that I can't remember right
now.

The button moves to the bottom-right corner under the view when the view shows at least
one item.

The explanatory string is also tweaked slightly to reflect the new view, and made into a
level 3 Kirigami heading like other similar messages.

Test Plan: {F8174853}

Reviewers: #vdg, #plasma, leinir, bshah

Reviewed By: #plasma, bshah

Subscribers: bshah

Differential Revision: https://phabricator.kde.org/D28040