Prettify sellable editor and subclasses

* Put details slave directly on SellableEditor, saving some space on
  ProductEditor for the extra details
* Create some sizegroups on SellableEditor and ProductInformationSlave so
  we can have widgets aligned without having to be on the same table (used
  by the details slave)
* Improve ProductInformationSlave to make it look nicer
* Put tax slave inside an alignment so it can have some padding

Change-Id: I2941aac47fff4dcff1655207cd1a5d586c82c69d