Fix assert when loading the python plugin

The python plugin shouldn't be a KisActionPlugin since it's loaded
only once and is owned by the application not the view manager;
KusActionPlugin shouldn't hard assert, since it checks carefully
whether the viewmanager is 0 in the rest of the code, however, it's
still wrong if it's created without a viewmanager as parent, so
do a KIS_ASSERT_RECOVER_NOOP(m_viewManager);.