Update EdgeItem after setting a new style

This fixes a missing update of the EdgeItem style
when setting a new EdgeStyle.

Fix For no Release Event Received

When certain custom actions like showing a QMenu
are done in a slot connected to the groupRightClicked()
signal which gets emitted in qanGroupItem::mousePressEvent()
the release event of the mouse button is never handled
by the QTQuick framework and e.g. the grabber item is
not cleared and will be re-used in the next mouse press
event.

This is fixed by moving the emit of 'groupRightClicked()'
to the mouseReleaseEvent() handler.

Fix for receiving QT signals in destructed object

When the container of incoming and outgoing nodes is cleared
in the destructor of a gtpo::node the destructor of the
derived class was already called and the nodes container
shall not send any 'length changed' signals which can be
received by the already destructed object of derived type.

Merge pull request cneben#133 from cneben/develop

Develop