Qt Callstack: Clear the callstack every time it's updated

This fixes the issue that old members of the callstack would stick around, even when the callstack shortened.

src/citra_qt/debugger/callstack.h

@@ -21,4 +21,7 @@ public slots:
 private:
     Ui::CallStack ui;
     QStandardItemModel* callstack_model;
+
+    /// Clears the callstack widget while keeping the column widths the same
+    void Clear();
 };