This commit adds a download item to the album and albumMulti
contextMenu functions. Since the Archive class is already general
enough, we can easily support to download multiple albums. This
is especially nice, because we can now select individual albums.
That is, albums can now contain other albums, which are shown at
the top of the album view. This required some changes to album.js
and the contextMenu.js, because this view contains now both
photos and albums.
The contextMenu on this view has been kept simple by requiring
the user to select either only albums or only photos, but not
a mixture of both.
This feature required a database change, so that the version
has been updated to 3.1.3.
At the moment, album and photo operations (make public, download,
delete, merge) are still "flat", i.e. don't respect the album
hierarchy.
