Is your feature request related to a problem? Please describe.
When the user clicks a <DeleteButton> in an Edit toolbar, react-admin displays "Element deleted". I would like to customize that message for different resources (e.g. "Post deleted" and "Comment deleted").

Describe the solution you'd like
I would like DeleteButton to behave like SaveButton and accept onSuccess and

For mails which are not a thread but just single emails, the layout is a bit strange currently cause it shows too much info. Also it makes no sense to be able to collapse the only email.

So we could check what to do with the header there (possibly just not show the people bubbles in the header?) and probably block collapsing the mail.

What I'm trying to achieve

Placeholder "Search ..." is only visible if focus is set to the field search, so it is not clear that this field is there.

Steps to reproduce the problem

1. Log in as an Admin to Dashboard.
2. Go to page where search is available e.g. Products
3. Move mouse to the field with search
4. Placeholder is shown after step no. 3.

What I expected to h