Attachment selector doesn't use the sourceDocument reference

The attachment selector always use the current document for listing the attachments and for resolving the references, which is wrong since we provide a sourceDocument to use for such cases, which is used for uploading attachments.

Reproduction steps:

• Create a page Test.XClass.WebHome containing a textarea property named "test" that uses WYSIWYG editor and Wiki Syntax
• Create a page Test.XObj.WebHome containing an xobject of type XClass previously created
• Create a page Test.Test containing the following piece of code:

  {{velocity}}
  #set ($myDoc = $xwiki.getDocument('Test.XObj.WebHome'))
  #set ($myObj = $myDoc.getObject('Test.XClass.WebHome'))
  $myObj.display('test', 'edit')
  {{/velocity}} 

• Upload an attachment in the attachment tab of Test.XObj.WebHome
• On the displayed editor click on "link" and select "attachment"

Expected result:

• the attachment selector is opened on page Test.XObj since it's where the xobject is stored
• if an attachment is selected from current page (Test.Test), it's resolved in taking into account that the xobject is stored in Test.XObj

Obtained result:

• the attachment selector is opened on current page
• when inserting a link for an attachment, it's resolved relatively to current page Test.Test