Fix suggested schemas added automatically upon saving a ui component

Steps to reproduce:

• Create a Page Provider and add a predicate to dc:title.
• In Designer, create a drawer menu item, name it and configure it to use the created Page Provider.
• On the schemas field, remove all the schemas (there should only be a dublincore entry) and add anything else but the suggested schemas.
• Save and notice that the suggested schemas are added back.

Note: even though the editor shows that these schemas are added after the save, they are not persisted in the bundle (unless the user saves again). This might be due to a problem in the initialization of the attributes editor.