Inventory window

I created the item window. You can move items with drag&drop (thanks, jQuery UI) and the server saves the positioning.

I also added the /item debug command to create item stacks.

Still missing:

• Showing item amounts
• Item information window
• Using items / dragging usable items to hotkey bar
• Splitting item stacks
• Equipment
• Make item window resizable (both visual size and size of grid)