Rename Item tab to Inventory

Besides "item" being one of the most ambiguous labels possible (every tab is involved with items of some type), the term for what is distinctively managed here is "Inventory", and has been for about 30 years now.

At first I thought that it was named "Item" because it was intended for something more or less general than inventory, but after using it and having several discussions on the forum about that tab, I believe that inventory management is the precise goal of this tab. Though the use case isn't completely implemented yet, that's no reason to disguise the purpose.

Then I was thinking that maybe there was a widget width concern and "Inventory" was too long, but surely if we can afford 11 letters for "Description", we can afford 9 letters for "Inventory". And if not, "Invent." or "Inv." would still be much more informative than "Item".