digikam from trunk:
Load tag thumbnails asynchronously, speed up startup

- add a new class AlbumThumbnailLoader and consolidate all
tag and album thumbnail loading code
- use this class in TagFilterView, TagFolderView, ImgDescEditTag
- do no longer use syncjob for tag thumbnail loading,
but use asynchronous mode for custom tag icons (as done for PAlbums)

Please check that this fixes the huge startup delays

TODO: Need to check that icons are actually loaded
only once by slave if added multiple times
(not relevant to startup speed)
CCBUG: 124688