[Plasma Windowed] Disable standard actions on SNI

We add our own "Close" item manually. The default quit entry would quit the application
but one Plasma Windowed instance can hold multiple applets each with their own SNI.

Also, add a separator (QMenu is smart enough not to show separators at the beginning of
the list, ie. if there's no applet actions)

Differential Revision: https://phabricator.kde.org/D5333