[System Tray] Do not send ContextMenu signal twice

Summary:
System Tray sends two "ContextMenu" events which breaks focus.
One is send on mouse pressed, second on mouse clicked. As a result
right click on the system tray icon causes focus to lock on the icon.
If later user clicks somewhere else event is send to the first icon.

BUG: 376277
BUG: 409768
FIXED-IN: 5.17.0

Test Plan:
The best to test with Skype for Linux with SNI bridge.

To reproduce:
* Right click on the Skype icon, click on menu item (can be Show Skype)
* Right click on any other icon in system tray (or even any other place on the panel)
* Skype menu will appear

After fix:
* Right click on the Skype icon, click on menu item (can be Show Skype)
* Right click on any other icon in system tray (or even any other place on the panel)
* Correct menu should show

Reviewers: #plasma, broulik, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: broulik, apol, plasma-devel, #plasma_workspaces

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22804