A placeholder is looked up:
1. in the field's options ("placeholder")
2. on the t-field node itself (@placeholder)
3. on the field's column (.placeholder)
The first one found is used as the @placeholder of the resulting node
(in-HTML), and displayed when the field is empty and unfocused.
If no placeholder is found, an empty field just gets a small padding
in order to be easier to see & focus.
bzr revid: xmo@openerp.com-20131218114901-xi3ye11x5pysq2a2