WebUI: Implement subcategories

[Bartuzen]

• Show subcategories in a tree view in WebUI
• Add Add subcategory... context option to WebUI
• Add use_subcategories property to maindata in WebAPI
• Add Use subcategories preference to WebUI & WebAPI

Closes #12819

[glassez]

You've just allowed to switch core handling. But aren't categories expected to be shown in a tree view when "Use subcategories" is enabled (just like it's done in the qBittorrent Desktop GUI)?

[Bartuzen]

Actually, my aim was just implementing the option. But I decided to try implementing tree view. I'll either update this PR or create a new one when I'm done.

[glassez]

But I decided to try implementing tree view.

Perhaps you could consider how torrent content view is implemented.

[thalieht]

GUI has context menu item "Add subcategory..." but i don't know if it's in scope of this PR.
Other than that i found no problems.

[glassez]

It does not display torrents from subcategories when some parent category is selected.
Also the categories are non-collapsible unlike GUI, but this is not a blocking problem for me.

[thalieht]

It does not display torrents from subcategories when some parent category is selected.

Maybe i'm misusing subcategories... but i wish that's how it worked in the GUI as well. For example a category "books" containing subcategories "history" and "math". I can't think of any reason i would want to see a list of those 2 subcategories together. Also because of that, if i care about filtering torrents by category, i can never assign them to a category that has subcategories.

[glassez]

For example a category "books" containing subcategories "history" and "math". I can't think of any reason i would want to see a list of those 2 subcategories together.

Any history book is a book as well as any math book is a book so when I select Books category I expect to see all the books despite their subcategories. That's how it was designed. This is what makes it different from the other mode (w/o subcategories), where "books" "books/history" and "books/math" are just 3 independent categories that are not related to each other in any way, except for the partial matching of the names.

[glassez]

@Bartuzen
Thank you!

[sneakblade]

I think this is what fixed #18333, thank you.