HaikuDepot : Refactor Screenshots

Rework the way in which the application deals
with screenshots so that there is a repository
of screenshots that is referenced from
identifiers on the package model and not that
the screenshot bitmaps are stored with the
model and not cleaned up.

Change-Id: I632def96852a518b3f6b1c8b81909de117bbbc54
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7296
Reviewed-by: Andrew Lindesay <apl@lindesay.co.nz>

HaikuDepot: Remove Custom List

Further removal of the use of custom list class.

Relates To #15534

Change-Id: I1d84b562b334e5e52ed4772bad3a6aea7b715562
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3657
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

HaikuDepot: resize window to largest screenshot of pkg

* When opening the screenshot window for a package, set the window
size to the largest dimensions of all the screenshots of this
package. Prevents it from resizing/jumping around when browsing
through the images.

HaikuDepot: delay indicating screenshot download progress

* Only show barber pole if screenshot download takes longer
than 200ms

HaikuDepot: add icons to screenshot window tool bar

* Arrow icons instead of text for previous/next buttons

HaikuDepot: begin support for multiple screenshots

* Initial support for displaying multiple screenshots for packages
which have more than one. Still rough and unfinished.

Screenshot window now has a toolbar with prev/next buttons and
a busy loading indicator. Switching through the screenshots works.

There's currently a server-side bug which makes all data turn up
15 times in the JSON file, so please don't report a bug about
HaikuDepot showing 15 or 30 screenshots available when it's really
just 1 or 2 :)

Still to be done: toolbar icons instead of text labels; better
handling of screenshot window resizing; maybe thumbnails of
screenshots and preloading other screenshots in the background.
Main window also needs a way to indicate that there are more
screenshots than the one thumbnail, needs some more thought about
how that might look.

This concludes my HaikuDepot commits from the coding sprint at
KDC 2017 Toulouse!

HaikuDepot: Improved screen shot window

* Black background
* Auto-resize to fit new screen shot, re-center on screen

HaikuDepot: Added rudimentary screenshot window.

Clicking the thumbnail in the package info area will now open a
window where the full-size screenshot is downloaded and shown. There can
only be one such screenshot window open. Navigating between multiple
screenshots of a package is not yet implemented. Also, no caching, but
this may be OK as it is, since one usually looks at these only once and
doesn't want the harddrive filling up with screenshots.

HaikuDepot: Improved screen shot window

* Black background
* Auto-resize to fit new screen shot, re-center on screen

HaikuDepot: Added rudimentary screenshot window.

Clicking the thumbnail in the package info area will now open a
window where the full-size screenshot is downloaded and shown. There can
only be one such screenshot window open. Navigating between multiple
screenshots of a package is not yet implemented. Also, no caching, but
this may be OK as it is, since one usually looks at these only once and
doesn't want the harddrive filling up with screenshots.