HaikuDepot: Store the screenshot info...

... from the bulk request results in each PackageInfo.
HaikuDepot/WebInterface: Implemented retrieving screenshots
HaikuDepot/SharedBitmap: Try load BBitmap via BTranslationUtils
HaikDepot: Lazily load screenshots.

The first one is displayed. Clicking the thumbnail does nothing,
as before. (There is a TODO in the code to open the bigger version.)
HaikuDepot/List: added IsEmpty() method
HaikuDepot: Make sure lists are cleared...

... in case the code runs multiple times for the same package-ref.
(Did not happen, but it's just better form.)