Don't tell the screen reader that a button is related to an image which has no useful label

This MR allows the application developer to override the A11Y label for buttons which have only a symbolic icon (e. g. the icon is set using set_icon_name). The A11Y relation there forces Orca to follow it and in all cases I observed the added value was just a GtkImage addition to the audible label, e. g. i never saw a case where the label would be semantically superior to an empty string.

This fixes #4258.