App: add class TransactionLocker

To protect some critical transaction from being closed prematurely. It
is currently used to protect transaction of object deletion.

src/App/Document.cpp

@@ -91,6 +91,7 @@ recompute path. Also, it enables more complicated dependencies beyond trees.
 #include <QCoreApplication>
 #include <QCryptographicHash>
 
+#include "AutoTransaction.h"
 #include "Document.h"
 #include "Application.h"
 #include "DocumentObject.h"
@@ -3764,6 +3765,8 @@ void Document::removeObject(const char* sName)
         return;
     }
 
+    TransactionLocker tlock;
+
     _checkTransaction(pos->second,0,__LINE__);
 
 #if 0
@@ -3865,6 +3868,8 @@ void Document::_removeObject(DocumentObject* pcObject)
         return;
     }
 
+    TransactionLocker tlock;
+
     // TODO Refactoring: share code with Document::removeObject() (2015-09-01, Fat-Zer)
     _checkTransaction(pcObject,0,__LINE__);