Inventory item section&description + validation

I added field to modify the section of an item and its description. The description is not visible in the inventory table. Therefore, I added an option to see the details of an item. So we have 2 ways to edit an item, a quick edit mode in the table and a separate page to modify it with more space and fields.

I also added/fixed some validation and error messages

<!---
@huboard:{"order":0.15625,"milestone_order":17,"custom_state":""}
-->

This technically should only use a localization sprintf format. Numbers don't come in the same order in all cultures, and even "#" is "no." in French for instance. Good enough for 1.0 I imagine, but we'll have to keep this in mind.