improved codestyle
code improvements
bigger cleanup of downloader
remove wait_time from download_queue

this was every 0 and also wrongly implemented
remove mutex to dl_list since it is not needed

there is just one instance of downloader
and nothing else has access to dl_list
simplifications for downloader
further downloader simplifications

removed display_pos
this was used to have always the same ordering when printing the
progress
with just console output this is not needed/possible
when it later is required the uid can be used to create a default ordering
rename dl_list to current_downloads and readd mutex

my guess about not needing that mutex was wrong - now I
documented the class a bit better