[Task Manager] Hide group dialog when opening applets settings

When moving focus from one Plasma window to another (e.g. in this case opening the
applet config dialog), Plasma Dialog stays open even with hideOnWindowDeactivate.

Since changing task manager settings might affect grouping and triggers unintentional
behavior when the model is used in a GroupDialog without grouping / fildering, just
hide the popup in this case.

Differential Revision: https://phabricator.kde.org/D4280