Better UI for the "Create a new vault" button

Summary:
The button to create a new vault now:
- has an icon "list-add"
- shows in the center of the applet when there are no existing vaults

BUG: 385972

Reviewers: #plasma, cryptodude, ngraham

Reviewed By: ngraham

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9045