Fix bugs when copying a page

Steps to reproduce:

1. Go to Designer > Tabs
2. Create a new "Page"
3. Set :
   • Name = testPage
   • Label = TestPage
   • Order = 1
   • Element : choose an existing one or create a new one
   • Attribute > Name : testPageName
4. Save work in progress
5. Go back to Designer > Tabs
6. Push the button "Copy" for the element "testPage"
7. The new Page is opened
8. Observe that the Name has been suffixed with _copy, and the attribute name too
9. Set a new Name and save

Observe the first bug : Label and Order have been cleared.

Then, go to nuxeo-xxx-bunde.html > second bug : observe that the "_copy" id is still used to reference the new page

<nuxeo-slot-content name="testPage_copy" slot="DOCUMENT_VIEWS_ITEMS" order="3">