[WIP] Advanced Inventory Advanced

This is a redo of the Advanced Inventory, using somewhat more modern and straightforward code.

Due to slight architectural changes, this entails some functional changes:

  - Items can no longer be moved into the "All" area; but items from the "All" area can now be moved all at once into another area.
  - Categories (not yet reimplemented) is no longer a sort option, but a separate setting, usable with any sort.
  - Moving all items between areas now respects the sort rule of the origin area.
  - Setting the filter string is no longer done by the advanced inventory, but by the standard input popup.
  - Sort options now follow the selected area, not the selected side of the window.

Note to the editor: Existing advanced inventory code is retained in order to facilitate merging: It will be removed.