Allow which metatags are visible on the field widget to be editable

I rerolled the patch to 2.0.0.
For now I'm not able to set the branch to 2.0.x and provide a merge request, hope someone else can do it.

This would be a welcome addition to the Metatags module! The forms are too long now and contain obsolete information:

• The Basic tags contain two fields that are no longer supported. This information clutters the interface.
• The Open Graph tags form is really long.

I mostly try to avoid printing the form for website editors (mostly by using a summary field) but sometimes they want a custom metatag description.

I'm currently hiding the unused metatags in Seven with some CSS (I use a small custom module to override the admin styling):

#edit-field-metatags-0-basic .form-item,
#edit-field-metatags-0-open-graph .form-item {
	display: none;
}

#edit-field-metatags-0-basic .form-item.form-item-field-metatags-0-basic-title,
#edit-field-metatags-0-basic .form-item.form-item-field-metatags-0-basic-description,
#edit-field-metatags-0-open-graph .form-item.form-item-field-metatags-0-open-graph-og-title,
#edit-field-metatags-0-open-graph .form-item.form-item-field-metatags-0-open-graph-og-description {
	display: block;
}

This will expose only the meta title and description on the basic and Open Graph fields. It's a dirty solutions but it helps to clean up the interface.

Changing the status as I have successfully applied this patch.