Add a price comparison condition

One of the challenges in trying to make the Rules UI more friendly to Drupal Commerce price data manipulation and comparison is that in order to convert major unit decimal values to minor unit integer values we must know the currency. However, in a Rules configuration form context, the currency could literally be anything. We can make a best guess by assuming that we'll only ever have to deal with enabled currencies, but even that could leave open an edge case where products have prices in currencies that are no longer enabled on the site.

One middle of the road solution is to actually add a price comparison condition that people can use to compare product prices, line item prices, and order totals. This condition can take a price field selector parameter, a decimal price field, and a currency code select list, and when the condition is actually evaluated it can use our currency conversion function to first convert the selected price to the currency chosen for the comparison. For single currency stores, instead of a select list, we'd just show the currency code as a suffix to the decimal textfield.