Qt Designer let's you add custom slots and signals to the main
form; they should use string-based connection syntax since the
class is not known in setupUI(). Amends
da3cb1deb6.
Task-number: QTBUG-76375
Change-Id: I5a3a5630f77c812d48db1cdb7a8658a4d2718228
Reviewed-by: Liang Qi <liang.qi@qt.io>