Fix heap-use-after-free in Part destructor

Tear down the ViewManager on the Part destructor instead of letting Qt destroy the child objects itself.

This way we control the sequence more and make sure we don't end up using memory that was already deleted. Fixes the ASAN detected problem in https://build.kde.org/job/konsole%20Applications-15.08%20stable-kf5-qt5/PLATFORM=Linux,compiler=gcc/12/testReport/%28root%29/TestSuite/PartTest/

Neither ASAN nor valgrind complain anymore when running PartTest.

REVIEW: 125366
(cherry picked from commit ace53ef4)