This commit makes the placeholder hint not be dependent on their only
being one textbox with placeholders on a page.
To do this it needs to be its own module, because the two textboxes are
each self-contained instances of the same module.
The way I’ve gone about this is to make the placeholder hint accept a
list of textboxes it should look in when deciding what message to
display to the user.
Most of the code is extracted from the existing placeholder highlighting
module.
