Introduce a "Prices" tab for variations

The attached patch adds a "Prices" tab + entity operation for variations (It'd probably make sense to have a similar tab for products as well), see screenshot below:

The "Prices" tab is provided by a view, which has a "Price list" exposed filter.

I wrote a functional test that tests the view and the entity operation.

I had to disable the view caching since the tests were failing when a second price list item was created for a variation and the "Prices" page was reloaded (the second price list item wasn't present on the page), there's probably a way to fix this, but could be implemented as a follow-up I guess.

I first tried grouping the prices by price list, but the result was kind of ugly, so decided to just sort the results by price list weight ASC + price list id DESC.

Unfortunately, there's currently no way to create a price list item from that page, since we'd need the price list id in order to do that.

Furthermore, Views doesn't provide a dropdown exposed filter for entityreference fields (See #2429699: Add Views EntityReference filter to be available for all entity reference fields), so I had to implement that via a hook_form_alter().

I also needed an argument default handler that wasn't present in Commerce (committed to core since, See #3093082: Create a views argument default plugin to extract a Product variation ID from URL), but I didn't want to require Commerce dev, or 2.16 for such a simple need so I copied the handler in Pricelist.

Now that we require Commerce dev, we don't need the argument handler anymore.

I renamed the view to "commerce_product_variation_prices", and no longer turning the price list into a dropdown if there are more than 25 price lists...

I tried to fallback to the #entity_autocomplete element in that case, but for some reason, the view returns no results when no price list is selected in the autocomplete element.

Attaching the interdiff.

Committed! I'll create 2 follow-up issues to allow adding prices from that page, and to implement a better "fallback" for the price list filter when there are more than 25 price lists.