Add support for "target" to link type Inputfield header actions

[Toutouwai]

A header action in a dropdown can define a target attribute for a link: https://github.com/processwire/processwire/blob/9a2e3c056e99d486866f2231bfa4d3923221d272/wire/core/Inputfield.php#L2154

But if the header action is not in a dropdown there is no option to define a target: https://github.com/processwire/processwire/blob/9a2e3c056e99d486866f2231bfa4d3923221d272/wire/core/Inputfield.php#L2109-L2115

This request is to add support for a target option so that header actions have consistent features whether they are in a dropdown or not.