HaikuDepot: SharedBitmap: Added loading from buffers.

 Refactored loading from a buffer when loading from resources and
 re-used that for instantiating a SharedBitmap from a BMallocIO.
HaikuDepot: Refactored setting title and icon in listview...

... so that it is also done when a package state change notification
is received that has the icon bit set.
HaikuDepot: Populate package icons from web app.

 * Transformed ConnectionTest into WebAppInterface
 * Added methods to Model to trigger the population of all packages
   with additional information that is needed in the list view.
 * This launches a thread which tries to retrieve icons for all known
   packages from depot.haiku-os.org. This is uncached and very slow. I
   guess it could even be fast enough with a change to the protocol where
   icons are not retrieved one at a time.