entry-row: Support placeholder text
It would be convenient to have placeholder text like in GtkEntry
as it's not really replaced by the row's title.
As an example, we need to redesign the Post Review dialog in Software, and the summary entry could be replaced by something like this:
Maybe the placeholder could be in italics too, and maybe it could show up all the time to match the layout of an AdwActionRow
with a title and a subtitle.