Prevent segmentation fault and construct DlgAddWatchEntry dialogs in the stack.

[cristian64]

Previously, these dialogs were constructed dynamically in the heap, and were never deleted explicitly; only when their parent QObject was destroyed at a much later time would they be deleted.

This deferred destruction was in fact masking a segmentation fault caused by a double-free: the entry in the dialog was not extracted via stealEntry(); instead the reference was taken from the entryCopy variable (same pointer), resulting in the entry getting deleted twice, eventually.

Reproduction steps:

• Double-click on an watch entry to bring up the Edit Watch dialog.
• Make any edit and press OK.
• Quit the application gracefully to force the destruction of the dialog.
• A segmentation fault would be produced:

(gdb) bt
#0  QArrayDataPointer<char16_t>::deref() (this=<optimised out>, this=0x558b5d1a35c0) at /usr/include/c++/11/bits/atomic_base.h:392
#1  QArrayDataPointer<char16_t>::~QArrayDataPointer() (this=0x558b5d1a35c0, __in_chrg=<optimised out>) at /usr/include/x86_64-linux-gnu/qt6/QtCore/qarraydatapointer.h:129
#2  QString::~QString() (this=0x558b5d1a35c0, __in_chrg=<optimised out>) at /usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1302
#3  MemWatchEntry::~MemWatchEntry() (this=0x558b5d1a35c0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/MemoryWatch/MemWatchEntry.cpp:48
#4  0x0000558b5c517787 in DlgAddWatchEntry::~DlgAddWatchEntry() (this=0x558b5d1be8f0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/Dialogs/DlgAddWatchEntry.cpp:31
#5  0x0000558b5c51782d in DlgAddWatchEntry::~DlgAddWatchEntry() (this=0x558b5d1be8f0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/Dialogs/DlgAddWatchEntry.cpp:32
#6  0x00007fde5556599b in QObjectPrivate::deleteChildren() () at /lib/x86_64-linux-gnu/libQt6Core.so.6
#7  0x00007fde5624cab8 in QWidget::~QWidget() () at /lib/x86_64-linux-gnu/libQt6Widgets.so.6
#8  0x0000558b5c51d0cd in MemWatchWidget::~MemWatchWidget() (this=0x558b5cd068e0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/MemWatchWidget.cpp:41
#9  0x0000558b5c5367cd in MainWindow::~MainWindow() (this=0x7ffc70ef10f0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MainWindow.cpp:56
#10 0x0000558b5c4f1ad7 in main(int, char**) (argc=<optimised out>, argv=<optimised out>) at /w/dolphin-memory-engine/Source/main.cpp:56