Translate a page dropdown tree

[jkevingutierrez]

Found a bug, please fill out the sections below

Issue summary

Having a big page tree, the "Parent page" dropdown could be messy. Especially, if there are pages with the same name as children of different pages.

It would be great to have something similar to the PageChooserPanel to select the parent page.

If not possible, it would be great to make more visible the hierarchy in the dropdown. Something like:

How to reproduce?

Having something like

/   (TranslatableSiteRootPage)
    en/        (TransHomePage)
        subpage-1/    (TransLandingPage)
            subsubpage-1/    (TransLandingPage)
                mypage/    (TransLandingPage)
            subsubpage-2/    (TransLandingPage)
        subpage-2/    (TransLandingPage)
            subsubpage-1/    (TransLandingPage)
            subsubpage-2/    (TransLandingPage)

    es/        (TransHomePage)
        subpage-1/    (TransLandingPage)
            subsubpage-1/    (TransLandingPage)
            subsubpage-2/    (TransLandingPage)
        subpage-2/    (TransLandingPage)
            subsubpage-1/    (TransLandingPage)
            subsubpage-2/    (TransLandingPage)

If I want to translate en -> subpage-1 -> subsubpage-1 -> mypage to es it won't be clear in the "Parent page" dropdown where is the es -> subpage-1 -> subsubpage-1

It would also be good to have a default selected element in the "Parent page" dropdown, or show a "recommended page" help text. Maybe the logic used in WAGTAILTRANS_SYNC_TREE to copy pages in the correct location, could also be used to detect the default or the recommended Parent page?

Technical details

Python 3.8
Django==3.0.9
wagtail==2.10
wagtailtrans==2.1