Use qOverload instead of static_cast inside connect

Summary: Since we are now using C++14 use qOverload which is more readable than the static_cast equivalent.

Test Plan: Spectacle still builds and has the same behavior as before.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D21095