tax widget shouldn't be applied to all price fields

If commerce_tax steps in and provides extra settings for price fields, then the default price field commerce_price can have that value as TRUE baked in. Then if commerce_tax module isn't enabled, that's just an extra value in the field info array that won't have any effect.

I guess if you don't want tax on the basic price field, you don't enable tax module ;)

And currently, enabling tax module sticks the tax widget there, so there's no regression. So fine by me :)

Actually, I think that related to #1508320: Prices are calculated confusingly on multi VAT tax rate environment we should provide a means to have a price field widget that doesn't include VAT available for any field, including price.

Hence I think the best fix for this would be to provide two widgets for price: one with the VAT subwidget and one without.

The plot thickens:

- Price module already provides two widgets, one with currency and one without
- The tax element of the widget is added in by form alteration (but see #1791226: use hook_field_widget_form_alter() instead of hook_field_attach_form() for adding tax to price field widgets for a patch that makes this a bit nicer DX)

So providing a separate widget would mean 2x2 widgets, which is pretty bad for architecture smell.

However, providing field settings for whether or not to add tax has problems too:

- the price field is locked, therefore we can't add them in to the main field edit page with 'field_ui_field_edit_form'
- 'field_ui_field_settings_form' is the wrong place to do this completely

Therefore I think the only place that this can be done in is http://api.drupal.org/api/drupal/modules!field_ui!field_ui.admin.inc/fun..., the widget type selection form.

Here, Field UI module does nothing for us, so we'd need a custom submit handler. It's hardly ideal but I can't think of anything better.

... though I see that the Commerce Price module already has settings for the field that in normal operation are completely inaccessible because the field is locked -- these are in commerce_price_field_widget_settings_form().

So it would be feasible to add some setting to the field admin form by adding a hook_form_FORM_ID_alter() for 'field_ui_field_edit_form', commerce_tax_form_field_ui_field_edit_form_alter(). These settings wouldn't be visible in the UI but users could unlock their price field and gain access to them, or change the settings directly in exported Features.