Prevent segmentation fault and construct DlgAddWatchEntry dialogs i…

…n the stack.

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
aldelaro5#1  QArrayDataPointer<char16_t>::~QArrayDataPointer() (this=0x558b5d1a35c0, __in_chrg=<optimised out>) at /usr/include/x86_64-linux-gnu/qt6/QtCore/qarraydatapointer.h:129
aldelaro5#2  QString::~QString() (this=0x558b5d1a35c0, __in_chrg=<optimised out>) at /usr/include/x86_64-linux-gnu/qt6/QtCore/qstring.h:1302
aldelaro5#3  MemWatchEntry::~MemWatchEntry() (this=0x558b5d1a35c0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/MemoryWatch/MemWatchEntry.cpp:48
aldelaro5#4  0x0000558b5c517787 in DlgAddWatchEntry::~DlgAddWatchEntry() (this=0x558b5d1be8f0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/Dialogs/DlgAddWatchEntry.cpp:31
aldelaro5#5  0x0000558b5c51782d in DlgAddWatchEntry::~DlgAddWatchEntry() (this=0x558b5d1be8f0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/Dialogs/DlgAddWatchEntry.cpp:32
aldelaro5#6  0x00007fde5556599b in QObjectPrivate::deleteChildren() () at /lib/x86_64-linux-gnu/libQt6Core.so.6
aldelaro5#7  0x00007fde5624cab8 in QWidget::~QWidget() () at /lib/x86_64-linux-gnu/libQt6Widgets.so.6
aldelaro5#8  0x0000558b5c51d0cd in MemWatchWidget::~MemWatchWidget() (this=0x558b5cd068e0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/MemWatchWidget.cpp:41
aldelaro5#9  0x0000558b5c5367cd in MainWindow::~MainWindow() (this=0x7ffc70ef10f0, __in_chrg=<optimised out>) at /w/dolphin-memory-engine/Source/GUI/MainWindow.cpp:56
aldelaro5#10 0x0000558b5c4f1ad7 in main(int, char**) (argc=<optimised out>, argv=<optimised out>) at /w/dolphin-memory-engine/Source/main.cpp:56
```

Source/GUI/MemViewer/MemViewer.cpp

@@ -497,9 +497,11 @@ void MemViewer::addByteIndexAsWatch(int index)
 {
   MemWatchEntry* entry = new MemWatchEntry();
   entry->setConsoleAddress(m_currentFirstAddress + index);
-  DlgAddWatchEntry* dlg = new DlgAddWatchEntry(true, entry, this);
-  if (dlg->exec() == QDialog::Accepted)
-    emit addWatch(dlg->stealEntry());
+  DlgAddWatchEntry dlg(true, entry, this);
+  if (dlg.exec() == QDialog::Accepted)
+  {
+    emit addWatch(dlg.stealEntry());
+  }
 }
 
 bool MemViewer::handleNaviguationKey(const int key, bool shiftIsHeld)

Source/GUI/MemWatcher/MemWatchWidget.cpp

@@ -381,10 +381,10 @@ void MemWatchWidget::onWatchDoubleClicked(const QModelIndex& index)
     else if (index.column() == MemWatchModel::WATCH_COL_ADDRESS && !node->isGroup())
     {
       MemWatchEntry* entryCopy = new MemWatchEntry(node->getEntry());
-      DlgAddWatchEntry* dlg = new DlgAddWatchEntry(false, entryCopy, this);
-      if (dlg->exec() == QDialog::Accepted)
+      DlgAddWatchEntry dlg(false, entryCopy, this);
+      if (dlg.exec() == QDialog::Accepted)
       {
-        m_watchModel->editEntry(entryCopy, index);
+        m_watchModel->editEntry(dlg.stealEntry(), index);
         m_hasUnsavedChanges = true;
       }
     }
@@ -499,9 +499,11 @@ void MemWatchWidget::onAddGroup()
 
 void MemWatchWidget::onAddWatchEntry()
 {
-  DlgAddWatchEntry* dlg = new DlgAddWatchEntry(true, nullptr, this);
-  if (dlg->exec() == QDialog::Accepted)
-    addWatchEntry(dlg->stealEntry());
+  DlgAddWatchEntry dlg(true, nullptr, this);
+  if (dlg.exec() == QDialog::Accepted)
+  {
+    addWatchEntry(dlg.stealEntry());
+  }
 }
 
 void MemWatchWidget::addWatchEntry(MemWatchEntry* entry)