[RFR] [BC break] Easier custom toolbar

[fzaninotto]

Problem

Removing the delete button from the Edit toolbar used to require to create a Toolbar component from scratch.

Refs #2319

Solution

With this PR, it's as easy as passing just the <SaveButton> as only child of a custom <Toolbar>:

const PostEditToolbar = props => (
    <Toolbar {...props} >
        <SaveButton />
    </Toolbar>
);

export const PostEdit = (props) => (
    <Edit {...props}>
        <SimpleForm toolbar={<PostEditToolbar />}>
            ...
        </SimpleForm>
    </Edit>

There is a slight BC break here: people with custom Toolbar for Edit views used to have the Delete button displayed even though it wasn't in their custom toolbar. With that change, the Edit button will disappear. I think, with proper warning in the release notes, we can assume this BC break (on a feature that is very young and probably not very widely used to date).

[palyvodaBoi]

bad solution (as usual)
Save button loses its 'disabled' functionality with custom toolbar

I can believe it's hard to add 'showDeleteButton' property