READY: Add tree-based view for torrent files in download dialog

[ichorid]

fixes #1396
fixes #6443

I had to try at least three different ways of adding progress bars to QTreeWidget. This one is the safest, it does not trigger the dreaded "QObject deleted" error. Drawing progress bars by using setItemWidget is still slow, so it is disabled for torrents with more than 1k entries.
The proper, fast and safe way to implement this kind of stuff is to move to use the Model-View architecture. However, that would prove too much work, at least for now.

Add download dialog (Note that sorting works, and that there is a counter for total selected files size and select/deselect all buttons are removed as unnecessary. "Download exists" label is removed too - the corresponding notification is now shown in the same "Loading metainfo" label)

UPDATE2:
I've tripped at least three different QT bugs while doing this 🤦 The whole thing is built from workarounds. We really need to switch to Model-View eventually.

[devos50]

Great addition!

I’m aware that this PR is still WIP but I took a bit of time to see how it feels and looks. Except for selecting/deselecting files in the tree view of the ‘files’ tab on the download page, it seems to be functional. I have a few (mostly stylistic) suggestions:

Tree view when starting a download:

• Can you make the ‘Included?’ Column auto-size to the required width or would this be too much work?
• There is no context regarding the numbers below the table (e.g., 3.3 MB / 889.0 MB). I understand that the first number is the total selected file size whereas the second number is the overall size of the download. This is not clear for users. We can add something like: Selected: 3.3 MB, Total: 889.0 MB.
• Should we add ‘select all’ / ‘select none’ buttons?

Tree view when going to ‘files’ when downloading a torrent:

• The QTreeView is not using the full page width.

• I’m unable to select a bunch of files and exclude them for download. I used to be able to do so by right-clicking. The selection also seems to be a bit broken though (probably because you’re using the notorious QTreeView).
• I would suggest adding a few pixels of margin to the top and bottom of each progress bar.

Asking for help: anyone willing to help with CSS styles for checkboxes and progress bars?

I don't think there's much work to be done there. Re-using the progress bar style of the main download list is definitely a good idea for now. I think using the default checkbox styles for now is ok. Customizing checkboxes is not really easy and requires us to make customized images for different states (e.g., checked/enabled, etc.). I would consider that an effort for a further iteration.

[ichorid]

Tree view when starting a download:

* Can you make the ‘Included?’ Column auto-size to the required width or would this be too much work?

Yep, there is an option for that. I'll try it.

* There is no context regarding the numbers below the table (e.g., 3.3 MB / 889.0 MB). I understand that the first number is the total selected file size whereas the second number is the overall size of the download. This is not clear for users. We can add something like: **Selected:** 3.3 MB, **Total:** 889.0 MB.

Good point.

* Should we add ‘select all’ / ‘select none’ buttons?

Select all / unselect all buttons are unnecessary because clicking the top-level checkboxes magically 🧙‍♂️ sets/unsets checkboxes for the whole subtree.

* I’m unable to select a bunch of files and exclude them for download. I used to be able to do so by right-clicking. The selection also seems to be a bit broken though (probably because you’re using the notorious `QTreeView`).

Well, you're not supposed to, really... Just check/uncheck the checkboxes - that is it. Selection is completely unnecessary in the case of "tree+checkboxes" solution: it requires the same amount of clicks to either click the checkboxes or first select them and then open the menu, etc.

[ichorid]

There is a 7-years old QT bug that forces everyone trying to customize checkboxes to use pictures instead of CSS colours:facepalm:

[ichorid]

retest this please

[drew2a]

This PR improves UX, thank you for your work.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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