fix a crash caused by bad disconnects during debug install cancellation

Summary:
the processFinished disconnect would not actually work, change it to
function pointer syntax instead which makes the disconnect work as expected

change to Qt 5.7's QOverload helper to pick the right overload for connects

fix connect indentation

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5084