Consistently use text "Configure..." for applet configure buttons

Summary:
Applets which need initial configuration display some centered button
to noth indicate the status as well as help the user to quickly get
to the configuration page.

Having consistent text on those buttons improves UX.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12313