Pressing enter in the "new property" textbox should add the property

Steps to reproduce:

1. Go to /xwiki/bin/view/XWiki/XWikiClasses
2. Create a class named "Test"
3. Go to /xwiki/bin/edit/XWiki/TestClass?editor=class
4. Next to "Add new property", type "foo"
5. Press enter

Expected results: The new property should be added as though the Add button had been clicked.

Actual results: The new property is discarded and a preview page is shown as though Alt+P had been pressed.