Stock Location Search Stock Quantities

Add search functions on Products' Real and Virtual Stock on Stock Locations.

The "Stock by Location" view that is accessed from a product shows the list of locations, each one with the Real and Virtual Stock for the product. However, there is no way to filter out the locations which doesn't contain the product so the view is cluttered with a lot of lines with a 0.0 quantity. This module adds search functions on the quantity fields.

Also, by default, a filter hides the locations without quantity at all.