HaikuDepot: Added the notion of package categories
 * Defined PackageCategory class (icon, label and internal name)
 * Each PackageInfo has a list of PackageCategories
 * Model defines global PackageCategories, referenced by PackageInfos
 * Added Model.cpp to files needing translation
 * Added categories to dummy package infos
HaikuDepot: Make categories also accessible as List in Model
HaikuDepot: Beginnings of supporting filters...
  ... to reduce the package list. No filters can be defined
  via the UI, but a DepotFilter is already implemented, although
  its performance will probably need to improve.
HaikuDepot: Implemented filtering by category
HaikuDepot: List.h: added IndexOf(), Contains() and Remove()
  * All of those take const ItemType& as argument.
HaikuDepot: PackageInfoList -> PackageList, added PackageState enum

 * Model has SetPackageState(). It adds or removes a PackageInfo
   to/from three internal lists, installed, activated and uninstalled
   packages.
HaikuDepot: Fleshed out the code to support the virtual categories