Fix crash with Memory Viewer enabled

Summary:
Steps to reproduce (tested on kubuntu 16.04):

1. Set some breakpoint
2. Run debug
3. Close Memory View
4. KDevelop crashed

Or simply run debug without breakpoint and get "hang-up" of KDevelop at the end of debug session.

The problem of crash was in incorrect memory operations - MemoryViewerWidget class creates MemoryView objects and sets them QObject* parent to self. This leads to situation when connected MemoryViewerWidget::slotChildDestroyed method is called after finish of MemoryViewerWidget destructor.

Test Plan: Tested with with master branch

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4542