Fixed the [S]ort button.
Tweaked the column-splitting algorithm.
Properly update the two ends of the ItemButton-Section association.
containerProto:RedispatchAllItems is now independant of UpdateButtons.
Container: promote "UpdateButtons" to "FullUpdate" if self.forceLayout is true.
Container: properly show/hide/resize the "to sort" section.
Container: the "to sort" section is now sticky.
Container: never show free slots in "to sort".
NewItems: made the "new item" flag item-based and persistent.
Container: do not existing button to the "to sort" section.
Stack normally-non-stackable items by their ids.