xs:alternative and xs:assert are not supported (document marked invalid) #1092

Steps to reproduce:

• Write an XSD
• Inside an element bring up completion popup and select xs:alternative
• Tag is highlighted as error indicating that xs:alternative is not allowed which is wrong: xs:alternative can only be used inside an element

The same for xs:assert:

• Write an XSD
• Inside a complexType bring up completion popup and select xs:assert
• Tag is highlighted as error indicating that xs:assert is not allowed which is wrong: xs:assert can only be used inside a complexType

In both situations the completion popup correctly proposes the tags to be inserted but the syntax checker rejects them as invalid. Furthermore XML validation/completion using such XSDs fail altogether since the XSD is incorrectly parsed.

These are though two important tags to create more complex XSDs allowing some kind of data validation/completion instead of only structure validation/completion.