Allow undoing also the last SQL execution performed in Execute SQL tabs

Some clean-up of the code, so there is only references to the undo
savepoint name in a single file.

See issue #3345.

src/MainWindow.cpp

@@ -1490,7 +1490,7 @@ void MainWindow::fileRevert()
 void MainWindow::undo()
 {
     if (db.isOpen()) {
-        db.revertToSavepoint("DB4S_UNDO");
+        db.revertToUndoSavepoint();
         refreshTableBrowsers();
     }
 }

src/MainWindow.ui

@@ -1271,13 +1271,13 @@ You can drag SQL statements from the Schema column and drop them into the SQL ed
     <string>&amp;Undo</string>
    </property>
    <property name="toolTip">
-    <string>Undo last change to a cell in the browser</string>
+    <string>Undo last change to the database</string>
    </property>
    <property name="statusTip">
-    <string>Undo last change to a cell in the browser</string>
+    <string>Undo last change to the database</string>
    </property>
    <property name="whatsThis">
-    <string>This action is used to undo the last change performed to a cell in the database browser. Redoing is not possible.</string>
+    <string>This action undoes the last change performed to the database in the Database Browser or in Execute SQL. Redoing is not possible.</string>
    </property>
    <property name="menuRole">
     <enum>QAction::NoRole</enum>

src/RunSql.cpp

@@ -148,6 +148,9 @@ bool RunSql::executeNextStatement()
                 // We have to start a transaction before we create the prepared statement otherwise every executed
                 // statement will get committed after the prepared statement gets finalized
                 releaseDbAccess();
+                // Allow later undoing of this single execution with a non-unique savepoint.
+                db.setUndoSavepoint();
+                // And set the unique savepoint (if not already set) for the full current transaction.
                 db.setSavepoint();
                 acquireDbAccess();
                 savepoint_created = true;

src/sqlitedb.h

@@ -129,6 +129,10 @@ public:
     bool releaseAllSavepoints();
     bool revertAll();
 
+    // Set a non-unique savepoint for the general undoing mechanism (undoing only last write).
+    bool setUndoSavepoint() { return setSavepoint("UNDOPOINT", /* unique */ false); };
+    bool revertToUndoSavepoint() { return revertToSavepoint("UNDOPOINT"); };
+
     bool dump(const QString& filename, const std::vector<std::string>& tablesToDump,
               bool insertColNames, bool insertNew, bool keepOriginal, bool exportSchema, bool exportData, bool keepOldSchema) const;
 

src/sqlitetablemodel.cpp

@@ -484,7 +484,7 @@ bool SqliteTableModel::setTypedData(const QModelIndex& index, bool isBlob, const
         return false;
     }
 
-    m_db.setSavepoint("DB4S_UNDO", false);
+    m_db.setUndoSavepoint();
 
     if(index.isValid() && role == Qt::EditRole)
     {