Split item creation and insertion logic [API BREAK]

[Elv13]

The current :add_item() method both create and insert items, this is undesirable for pooled menu types. This is something I want to do since a while, but I avoided it not to break the API (breaking API suck), but I guess it cannot be avoided forever. I propose a new tree structure to cleanup the file tree:

   root 
     |->init.lua, base.lua, context.lua and all other
     |->layout
     |->style
     |->item
          |->init.lua (with some of the current base.lua code)
          |->layout (currently item_layout)
          |->style (currently item_layout)

So anything using item_layout or item_style will need to move to item.layout and item.style. Any objections?

[Elv13]

Fixed