Add googlemapfield class name so JS actually binds to the fields

[madmatt]

Not sure if this is just my implementation, but this module doesn't work out of the box with SS5 without the following:

• Add 'googlemapfield' class to the field (and not to the fieldholder)
• Remove :visible psuedo-class trigger as the Google Maps callback function often triggers before the page is rendered (meaning the field is sometimes not yet visible as the HTML isn't rendered).
• Only push the data for the specific field through when calling ->setValue() (see below).

This does remove extraClasses from the holder template, not sure if this is a bad idea or not really - but the JS breaks without this as multiple elements match the .googlemapfield selector but only the inner field has the relevant data- attributes.

Test case for the 3rd change:

1. Submit the form normally, see that all data processes fine.
2. Submit the form again, but this time miss a different required field with JS disabled.
3. This results in a server validation message for the other field, and results in the map not loading correctly (what actually happens is the data-options attribute contains coordinates of [null, null] which GMaps doesn't parse, so a map isn't displayed).

With this fix in place, data is saved and restored correctly when validation errors on other fields occur, and submission still works fine.