Allow user to choose 'target' attribute value

Hello,

It would be useful, for some websites, to be able to set the value of the attribute "target" of the links generated by Link module.
The attached patch provides a new radio button to "Link target" setting : "Manual target value", and a new form element "Manual link target value", with options "Allow the user to choose", and "Static target".

New possibilities are then provided to administrators : a link field can have a target attribute, different from '_blank' and 'top', and this value can be set by administrators (static), or by authors (when editing a node).

The patch does not break existing websites and data.