Set focus on widgets in system tray popup

Summary:
Fixes the issue most noticable with klipper's text field not having
focus; though use of debug rectangle showed it affected all applets.

Plasma popups get focus, which is given to our StackView, because that's a
focus scope that will give activeFocus to the focussed item within that scope.
However, nothing was explicity set, so nothing got focus.

Test Plan:
Debug rectangles followed by checking the original bug of opening klipper twice and on
both occasions typing immediately went into the text box

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3988