support for hidden google maps

Google Maps can not initialize if it is not visible since `$.entwine` does not support
the `:has` selector, and we do not know in which tab the map is located.

The only way to get this working (afaik) is to re-init the current tab on every tab switch. To limit the
performance loss, the `init` function will only act on `GoogleMapField`s in non-hidden tabs.