Improve structure of AddNewTorrentDialog code

[glassez]

Code restructuring that separates the basic logic from the logic that depends on the parameters and properties of the torrent being added.

[glassez]

@qbittorrent/bug-handlers
"Add new torrent dialog" behavior should not be affected.

[thalieht]

When was this functionality removed (doesn't work in master, works in 4.6.4) and why isn't it in the change log?

[glassez]

When was this functionality removed (doesn't work in master, works in 4.6.4) and why isn't it in the change log?

@thalieht
Good catch!
It was unintentionally added into this PR.
The fact is that it was originally a preliminary refactoring before other (deeper) changes. But they are still stuck indefinitely, so I decided to merge this commit separately. However, this part of changes doesn't really make sense yet, so I reverted it.

[thalieht]

Well i still don't know if it's meant to be removed eventually but right now it still doesn't work, this is not present in master:

qBittorrent/src/gui/addnewtorrentdialog.cpp

Lines 490 to 493 in 21143a9

	void AddNewTorrentDialog::setEnabled(const bool value)
	{
	settings()->storeValue(KEY_ENABLED, value);
	}

[glassez]

Well, it looks like you discovered a regression of #19355 along the way.
Fixed.