Wrong behavior when deleting a page with its children

I. There are missing pages in the selection tree when choosing the option to "Affect children" :

To reproduce this issue :

1. Create a range of pages in the Sandbox space, from Page-0 to Page-20
2. Go to Sandbox.WebHome, choose to delete the page
3. Select the "Affect children" option
4. Open the tree of "Pages that do not belong to any extension"
5. Click on "N more" to expand the tree

Expected result : all the child pages are being displayed in the tree

Actual result : some child pages are missing from the tree. In the example below, Page-3 is missing :

This means that it is impossible to delete a page with all its children.

II. When selecting "Pages that do not belong to any extension", the pages that are missing from the tree (see point I. above) are being deleted at confirmation.

When looking at the logs of the delete job, some documents are being displayed twice. With the example above, where Page-3 is not displayed in the selection tree, Page-12 shows up twice in the delete logs :

It turns out that Page-3 was actually deleted, so the duplication is probably a wrong display of Page-3.

III. In the tree of "Pages that do not belong to any extension", there actually ARE pages belonging to extensions (see Sandbox.TestPage1, Sandbox.TestPage2 and Sandbox.TestPage3). The reason why they're displayed in that section is that they're set to not be permanent, users being able to delete them (see https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-sandbox/pom.xml#L57-L68) . The name of this tree is therefore misleading and it should be replaced with something representing better that category of "unprotected" pages.