Fix cells copying (#825)

With this commit, we start to handle empty and NULL single cell
selections in a special way. Well, there's nothing special about it.
We just push them to inner buffer and clear outer clipboard. That's it.

NB: possible side effect might appear if you copy empty value,
then select some text and paste your empty value here. Nothing will
happen with text though, because, well, we wiped clipboard.

Plus, it fixes single-cell multiline text copy-pasting (#541)

src/ExtendedTableWidget.cpp

@@ -88,6 +88,8 @@ void ExtendedTableWidget::copy()
 
     SqliteTableModel* m = qobject_cast<SqliteTableModel*>(model());
 
+    m_buffer.clear();
+
     // If a single cell is selected, copy it to clipboard
     if (indices.size() == 1) {
         QImage img;
@@ -97,13 +99,23 @@ void ExtendedTableWidget::copy()
             qApp->clipboard()->setImage(img);
             return;
         } else {
-            qApp->clipboard()->setText(data.toString());
+            QString text = data.toString();
+            if (text.isEmpty()) {
+                // NULL and empty single-cells are handled via inner buffer
+                qApp->clipboard()->clear();
+                QByteArrayList lst;
+                lst << data.toByteArray();
+                m_buffer.push_back(lst);
+                return;
+            }
+
+            if (text.contains('\n'))
+                text = QString("\"%1\"").arg(text);
+            qApp->clipboard()->setText(text);
             return;
         }
     }
 
-    m_buffer.clear();
-
     // If any of the cells contain binary data - we use inner buffer
     bool containsBinary = false;
     foreach (const QModelIndex& index, indices)