Fix two crash bugs, add new context menu items

[camila314]

• Fixed a bug where opening a context menu with nothing selected crashed the program
• Fixed a bug where adding pointer levels could crash the program
• Created new 'Add Watch' and 'Add Group' context menu items when not selecting anything

[cristian64]

The first of the crashes was introduced [by me] in #171. The stack trace is:

#0  MemWatchTreeNode::getEntry() const (this=0x0) at /w/dolphin-memory-engine/Source/MemoryWatch/MemWatchTreeNode.cpp:61
#1  0x000055b08f3e877d in MemWatchModel::getEntryFromIndex(QModelIndex const&) (index=...) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/MemWatchModel.cpp:122
#2  0x000055b08f3f6c70 in MemWatchWidget::onMemWatchContextMenuRequested(QPoint const&) (this=0x55b091538000, pos=...) at /w/dolphin-memory-engine/Source/GUI/MemWatcher/MemWatchWidget.cpp:277
#3  0x00007f6377b6b023 in  () at /lib/x86_64-linux-gnu/libQt6Core.so.6
#4  0x00007f6378836889 in QWidget::customContextMenuRequested(QPoint const&) () at /lib/x86_64-linux-gnu/libQt6Widgets.so.6

I have not been able to reproduce the second of the crashes.

On a side comment, it would have been nice to keep the crash fixes in a PR, and the other enhancement on its own PR.

[dreamsyntax]

Clang format check failed
https://github.com/aldelaro5/dolphin-memory-engine/actions/runs/10548607990/job/29222766031#step:3:26

Run clang format and adjust the code it calls out in the above job