Preserve edits for multiline editor when length exceeds field size

Previously when using the multiline option for text edit widgets
the entire contents of the field would be discarded if the entered
value exceeded the maximum length for a string field.

Now the entered string is truncated to the maximum field length.

(cherry-picked from 7d8fba8)

Author: nyalldawson

src/gui/editorwidgets/qgstexteditwrapper.cpp

@@ -66,9 +66,19 @@ QVariant QgsTextEditWrapper::value() const
 
   QVariant res( v );
   if ( field().convertCompatible( res ) )
+  {
     return res;
+  }
+  else if ( field().type() == QVariant::String && field().length() > 0 )
+  {
+    // for string fields convertCompatible may return false due to field length limit - in this case just truncate
+    // input rather then discarding it entirely
+    return QVariant( v.left( field().length() ) );
+  }
   else
+  {
     return QVariant( field().type() );
+  }
 }
 
 QWidget* QgsTextEditWrapper::createWidget( QWidget* parent )