PR #17814.
Closes#17792.
Closes#929.
(Actually it should close all issues about lack of ability to stop torrent after metadata downloaded or after files are initially checked.)
Also makes explicit the temporary start of the torrent in the case when recheck of the stopped torrent is performed.
Change "Incomplete/temp folder" term with "download folder".
Allow to set "download folder" per torrent (in manual mode) and per category (in automatic mode).
Implement ResumeDataStorage class as base for all "resume data" storages.
Implement BencodeResumeDataStorage class and put all existing logic of
"resume data" loading/saving there.