This adds a new gui widget QgsFieldValuesLineEdit which includes
an autocompleter populated with current field values.
The autocompleter is nicely updated in the background so that
the gui remains nice and responsive, even if there's millions
of records in the associated table.
It's now used as a search widget for text fields, so can be seen
in the browser window if you set the filter to a text field, or
if you launch the form based select/filter by selecting a layer
and pressing F3.
