fixes for plasmawindowed sni support

Summary:
* support one sni per applet
* fix commandline parsing when the process is already running
  (--statusnotifier paramenter was breaking parsing)
* sync applet and sni status
* toggle shown/hidden based on current window status
* close action closes the plasmoid, not the whole app
* applet contextual action in context menu

Test Plan:
tested with different applets, more than once,
context menus correctly synced, status correctly synced
can quit plasmoids one by one

Reviewers: apol, davidedmundson

Reviewed By: davidedmundson

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5242