Make selected post-title unselect when another block selected

[kamataryo]

Fix for #1390

[kamataryo]

Previous:

This patch:

[youknowriad]

This is more complex than it seems, try saving you'll notice the permalink modal, and the title should remain selected (with borders) when we hit the "copy" button from the permalink modal.

[kamataryo]

@youknowriad Thanks for the comment. I fixed this PR and make the post-title check whether blocks selected or not and not attach is-selected class if so.

[mtias]

So here's a thought. What if we convert this to be a real block again, with the restrictions that:

• it's already inserted by default.
• you cannot move it.
• it doesn't show in the inserter.
• (bonus) you can hide it in the inspector to get an "aside".

This should give us the interactions for free, as well as pave the way for cusotmization where you will be able to do more stuff with these non-content pieces (meta data, title, date, categories, etc).

[gziolo]

#1390 was fixed in other PR. I'm closing this one. Thanks for spending your time on this PR :)