public void set_placeholder_text (string? text)
Sets text to be displayed in this when it is empty and unfocused.
This can be used to give a visual hint of the expected contents of the Entry.
Note that since the placeholder text gets removed when the entry received focus, using this feature is a bit problematic if the entry is given the initial focus in a window. Sometimes this can be worked around by delaying the initial focus setting until the first key event arrives.
a string to be displayed when this is empty and unfocused, or null