Implement inventory movement.

[Gavin]

So that Inventory Items may be easily checked out via the task inventory tab.

Movement logic is now possible but user interface still needs to be improved possibly using a webflow. Turns out a webflow was not suitable due to the way search pane works. Easy to create our own flow though. Done in r223, r224, r225, r226.

A task is required for movement type "Used" but not for other movement types such as "Correction" and "Purchase Received".

Movement "by current user" should also be recorded. Done in r217.