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.
Originally, it was just a wrapper for libtorrent::torrent_handle class, so it mimicked its name.
It was then transformed into a more complex aggregate, but the name was retained (just by inertia).
Unlike libtorrent::torrent_handle class in whose name "handle" means the pattern used,
it does not matter for qBittorrent classes and just eats up space in the source code.
Model returns string for DisplayRole.
Text alignment is set by Model (using TextAlignmentRole).
Delegate performs custom painting only where necessary
(i.e. for Progress bar).
Previously lookup is O(n), add operation is O(n), remove operation is
O(n).
Now lookup is O(1), add operation is O(1), remove operation is O(n).
n is the number of torrents already recorded.
