Add ability to "hide" item rows with zero-quantities

[armahillo]

@SCDB has said that they would like the ability to hide item rows that have zero quantities, because these items are often things that they don't deal with, and it just clutters the screen.

We should have a button (defaulting to "hide") that allows you to display those hidden rows. I'm imagining this would be done in the View layer (apply a class to the tr or something?)

Be sure to check that exporting as PDF doesn't include hidden data.

[seanmarcia]

Closing because issue #262 covers this.