[KDEPlatformSystemTrayIcon] Recreate deleted menu

Summary:
Recreate internal menu when needed.
It is possible that internal representation of system tray menu is
deleted. This can happen when QSystemTrayIcon is hidden.

BUG: 365105

Test Plan: Unit tests included

Reviewers: apol, davidedmundson, #plasma, #frameworks, broulik, nicolasfella

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24843