Use appropriate icon for unconfigured applets' "Configure..." buttons

Summary: CCBUG: 412114

Test Plan:
{F8116018}

(The icon size is too large by default but that's not a bug in this patch)

Reviewers: #plasma, davidedmundson, meven, ndavis

Reviewed By: meven, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27530