Add a refresh button to the plasmoid

Summary: This adds a refresh button to the plasmoid if no device is connected.

Test Plan: Refreshing works, and the button only appears when no devices are connected.

Reviewers: KDE Connect, nicolasfella

Reviewed By: KDE Connect, nicolasfella

Subscribers: apol, nicolasfella, KDE Connect

Differential Revision: https://phabricator.kde.org/D11501