Fix konsolepart segfault when closing after showing context menu

Assign the _view as the parent to the KXMLGuiFactory because the
factory is referencing the view widget as its associated widget. Since
the TerminalDisplay gets destructed first this is now a dangling
pointer. If the view is set as the parent the factory gets cleaned up
correctly. Also cleanup the created clientBuilder after destruction
because it can't have a parent and would probably leak memory.

BUG: 415762
FIXED-IN: 20.08.0
See also !87

(cherry picked from commit 1d7142ed24ef370ae984c0441d5b325b42656bd7)