This works fine here.
remains to do the js for the other core components (weblinks should be done in its own repo)

Ah, yes, you're right... Will look for which other components we have similar views and fix those too...

What about the string talking about "Subcategoires"?

PR for weblinks is joomla-extensions/weblinks#21

Thanks. Let's leave the string alone imho.

One more tester.

@smz @infograf768 I just have tested with success for com_content and com_contact, but not for com_newsfeed. Can this be regarded as successful test of this PR?

@smz @infograf768 P.S.: I mean I just did not test for com_newsfeeds, so I don't know if for these it is ok, too, or not. But a code review should show that the changes for these are the same as the (succesfully tested) changes for com_content and com_contact, so maybe my test is sufficient.

@test Tested now with success for com_content, com_contact and com_newsfeeds.
@smz It would be perfect if there was some tool tipp telling what the new +/- button is good for, if this is not Joomla! standard somehow so it is clear or not mentioned in docmuments.

@richard67 Richard, it is good that you tested this patch with com_contact and com_newsfeed too as... I actually didn't!!

The tooltip Idea IMHO is a good one, but probably I will write it down in the "TODO" list and leave it for the next fix release (so much to do now...)

<rant>I think com_contact and com_newsfeeds should burn in the JED hell together with com_weblinks. I think this is planned for 3.5 (but I may be wrong...). As far as regards com_newsfeeds I'm really tempted to plant a major bug in it and see what happens! Maybe a totally broken front-end functionality and, in the back-end, a jester popping out from the screen saying "Gotcha! This is a trick... press the button here below to restore full functionality!" Of course the button should also ping a joomla.org server so we can tally how many in the fields are actually using this... Am I too mean?</rant>

@smz Just a question: What do these "rant" tags mean? ;-)

@richard67 a rant... is vehement protest!

@smz Seems the issue tracker does not support this tag yet or is is meanwhile HTML5-Standard?

ehehehheheheh!! HTML6!!!

Thanks. Merged.

@infograf768 There is something strange with this merge: I see it as "Closed" (not "Merged") and I also have this message:

Closed with unmerged commits
This pull request is closed, but the smz:CategoriesLevelAll branch has unmerged commits.

@smz I see the same for my PR #5331 . Checking with staging has shown that merge was ok, and for your PR here it seemded, so, too (brief check only with bcompare). Unmerged commits could be in my case some in the middle of work for code style. So I am only confused about status = closed and not merged, but not about the unmerged commits.

... I'm pretty sure I didn't make any mod/sync to this branch... curious....

well, my mistake, have corrected my comment above meanwhile in github but issue tracker might show still old version. was not synch commits i had, was code style stuff for travis. maybe these intermediate steps have not been counted as comitted somehow and so make this strange unmerged commits message?

That message means the PR got merged, but not directly as a merge-commit. Instead we pulled the commits and squashed them, and then commited.
You can see the resulting commit here: cf88e6b