Select all subpages when checking parent page

[timobrembeck]

Motivation

In the page tree, typically a parent page should include all its subpages for all bulk actions.

Proposed Solution

When selecting a parent page in the page tree, all its sub-pages should be selected as well. This also applies to de-selecting a page. After that, it should be possible to (de-)select individual subpages without changing the selection status of the parent page.

Alternatives

Additional Context

[svenseeberg]

Basic concept: when executing a bulk action (XLIFF & PDF export), the view needs to check which checkboxes/pages are in the HTTP Post variables and which pages actually exist. All existing pages that are not in the HTTP POST inherit the "checked" status from the parent that is in the post.

We could create a decorator that can be applied to bulk action views/methods. This decorator could expand the post variables to all objects in the tree.

We have one special case to consider: if children are already loaded but collapsed again. To address this, we need to send a "collapsed=(True|False)" along for the parent node that indicates how to handle the check boxes that are not visible to the user but exist in the POST.