Set system tray applet to RequiresInput when a popup is showing

Summary:
This matches the base CompactApplet in plasma-desktop.

This prevents the panel autohiding when a popup is still showing.

BUG: 364847

Test Plan:
Opened popup panel didn't autohide.
Autohid once popup closed

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2034