More cleverly handle `xdg-user-dirs`-style folders using `QStandardPaths`

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## Problem

Currently, while Nextcloud Desktop does support multiple folder connections, it seems to be set up around the assumption that a user will just sync the folder located at `~/Nextcloud`.

If a user manually creates folder connections for each of the `xdg-user-dirs`-style folders (e.g. `~/Desktop`, `~/Documents`, and so on) while also syncing all other items within `~/Nextcloud` (i.e. the WebDAV root), Nextcloud Desktop will repeatedly warn that the XDG-style folder are subfolders of an existing folder connection.

Additionally, the user will have to manually deselect each of the `xdg-user-dirs`-style folders from the `~/Nextcloud` folder connection in order to prevent duplicate connections (what the above warnings are concerned about). Because this is a manual process, it is somewhat prone to user error, so duplicate folder connections may very well occur.

## Proposed Solution

Qt provides [a class called `QStandardPaths`](https://doc.qt.io/qt-5/qstandardpaths.html) for automatically determining standard paths including the `xdg-user-dirs`-style folders stated above on all the platforms Qt supports (i.e. not just XDG-compliant ones). Nextcloud Desktop could offer to create folder connections with each of the `xdg-user-dirs`-style folders using a simple checkbox and then automatically exclude those folders from the default `~/Nextcloud` folder connections.

To elaborate: the onboarding wizard could offer a list of `xdg-user-dirs`-style folders with checkboxes *before* the manual folder-connection step, and any folder connections created in this step would have their remotes hidden from the manual (i.e. usually the default `~/Nextcloud` connection) step that follows (with a checkbox to optionally show and allow duplicate connections). This process would be similarly available with the mini-wizard for "Add folder connection", and if the user were to add an `xdg-user-dirs`-style folder connection after the fact using the mini-wizard, the wizard would offer to switch the new connection from any existing parent connections in order to avoid duplicates.

This process would require a small number of new user-interface designs.

## Related Issues

Similar changes in Nextcloud Server could facilitate the above process. That is, Nextcloud Server could itself implement `xdg-user-dirs`-style folders such that the remote would more closely resemble the local user folder, with the `~/Nextcloud` folder actually being a subfolder of the remote `~`.

Implementing this in full compliance with [the `xdg-user-dirs` standard](https://freedesktop.org/wiki/Software/xdg-user-dirs/) (possibly even porting [the underlying library](https://gitlab.freedesktop.org/xdg/xdg-user-dirs) from C to PHP) could also facilitate consistent localization (a key feature of `xdg-user-dirs`) while avoiding conflicts.

Additionally, following XDG standards both locally (using `QStandardPaths`) and remotely (by porting `xdg-user-dirs`, among others) could facilitate adding more clever cloud synchronization for individual applications' data folders, i.e. using application icons, etc. in the main interface, but this is kind of a separate issue.

## Conclusion

This isn't just an offhanded suggestion; it's a persistent source of frustration for me, and I have, indeed, misconfigured folder connections in the myriad times I have set up Nextcloud Desktop. (In particular, fact that Linux uses `~/Videos` while macOS uses `~/Movies` has caused me to have separate folders with each name in my Nextcloud remote.)

I could create a related issue over at Nextcloud Server, but I have yet to do so.

Also, I just woke up, so while I have proofread this (and I've been thinking about the issue for the better part of a year), I can come back and add more illustrations (i.e. screenshots) and commentary.