HaikuDepot/SharedBitmap: switched to use BPositionIO...
...instead of BMallocIO specifically.
HaikuDepot: Implemented simple icon-cache.
 * In a first round, icons are trying to be loaded
   from the icon cache.
 * In a second round, the cached icons are updated from the
   server, unless the icons files have been modified within
   the last hour. The web-app does support headers for checking
   if the resource has been modified since a certain date, however,
   with hvif being so small and the time it takes to connect to the
   server per individual icon being so relatively long, it doesn't
   seem worth it just yet to optimize for not actually getting the
   icon data in response.