listview: Add GtkListItemManager

listview: Add GtkListItemManager

It's all stubs for now, but here's the basic ideas about what
this object is supposed to do:

(1) It's supposed to be handling all the child GtkWidgets that are
    used by the listview, so that the listview can concern
    itself with how many items it needs and where to put them.
(2) It's meant to do the caching of widgets that are not (currently)
    used.
(3) It's meant to track items that remain in the model across
    items-changed emissions and just change position.
(2) It's code that can be shared between listview and potential
    other widgets like a GridView.

It's also free to assume that the number of items it's supposed to
manage doesn't grow too much, so it's free to use O(N) algorithms.