Speed up startup and introduce progress bars

Ahoi!
Commit cea68db introduces progress bars while scanning for files and a spinner while initially discovering albums. This fixes issue #102 .
Commit d69bc98 shifts the parsing of the exif-data for sorting to later in the process, so that, if you don't want to sort, things are sped up dramatically (from ~3 minutes to some seconds on my 15k image library).  Additionally this way the image count is known and a progress bar can be displayed.
ok, but I think it's fine to remove it. The only difference is that the signal `album_initialized` will be called when the album is not sorted, but we could add a `album_sorted` at the end of `sort_medias`.