For some time, we've seen this being done in many text entries: the search entry in firefox, the search field in evolution, etc. Having a placeholder text, that is only visible when the entry is not focused and empty and acts only as an indicator of the nature of the input expected, looks like an interesting feature to have in GtkEntry.

I will attach an initial patch to implement this. It works fine but in RTL environments seems to break. What still needs to be improved is the positioning of the text layout, as this is currently (ab)using the position for the normal text. I'd appreciate some guidance on this in order to finish the patch.