[dialog] Align header to the left in case if no title is provided

[web-padawan]

Describe your motivation

Originally requested by #3654 (comment) and #3654 (comment)

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I would not change the right alignment if used with the title attribute where we can probably 99% agree that the added header information has to be right aligned.

My "problem" comes with the other cases where people wanna use the header with multiple components in a renderer like a flex layout with something like "icon, hx, right aligned buttons" where you don't use title on purpose to allow elements inside the header to be the title. Now you have to make sure to align it to the left on every Dialog you create.

Describe the solution you'd like

Align the header to the left by default and align it to the right if used in conjunction with the title.

[jouni]

Sounds like the right approach to me.

[DiegoCardoso]

For the Flow counterpart, we are creating a root element on the server, so that all elements are appended to it. So, essentially, there's a container between the header/footer parts and the child elements added to those parts. I wonder if using display: contents to these container can be used in order to avoid replicating the same behaviour on Dialog for Flow.