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.
Reduce the total startup time of the application and maintain sufficient responsiveness of the UI during startup due to the following:
1. Load resume data from disk asynchronously in separate thread;
2. Split handling of loaded resume data in chunks;
3. Reduce the number of emitting signals.
PR #16840.
Change "Incomplete/temp folder" term with "download folder".
Allow to set "download folder" per torrent (in manual mode) and per category (in automatic mode).
