Make tax calculations based on appliedcost

[DNNMonster]

I believe the line items are always being taxed based on their full price(totalcost) and this change should reflect any discounts that are applied to the item(appliedcost). I did test this for a single transaction which was accurate but it should be tested more thoroughly with vouchers, dealer discounts in addition to sales. I think the appliedcost is the right element to be using but you'll want to triple check that.

Potential fix for #95.

[DNNMonster]

I think we can potentially drop the isdealer check as well..?? I assume the applied cost will reflect the dealer pricing already but it's a case scenario that has to be tested.

[leedavi]

I'm not sure about the appliedtotal, I think this includes shipping and the shipping in France does NOT apply tax on B2C, it's does apply tax on B2B but only certain parcels. However the basic shipping provider does NOT calculate tax on shipping, Which I think is OK for France, but maybe NOT for other countries. Maybe we should have an option on that to turn tax on or off for the provider.

Maybe we should take the time and create some examples, with tax included and tax excluded.

[leedavi]

P.S. Thank you very much for looking at this.

[DNNMonster]

Closing this since this is included in another PR in progress which includes shipping tax changes.