Disable standard actions on VirtualKeyboard SNI

Summary:
We don't want a context-menu entry to quit KWin.

T5168

Test Plan: No more context menu shown.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4299