Merge SellableSearch and AdvancedSellableSearch

Merge both to reduce code duplication and make it easier to add
functionalities on both. ProductSearch should probably inherit from
SellableSearch too one day.

Change-Id: I354ad03eb18e742e76eb185ee06c3c99cce2a92a
Add option to show ImageViewer on SellableSearch

With this option, SellableSearch and all of it's subclasses will be able to
open the ImageViewer to take a look on the item's image they are going to select

Change-Id: I3dcc9e7ecc21fdab07446c87ff18025f48061e46