Prevent: Same default sync folder for multiple accounts

[TheOneRing]

Steps to reproduce:

• Add an ocis account
• Add a second ocis account, use advanced settings and use the default sync root form the first account

Both accounts use the same syncroot and differentiate the directory names inside that syncroot:

• root/Personal
• root/Personal (1)

Expectations:
The client prevents the user from using the same syncroot directory, instead offering a different syncroot:

• root/Personal
• root (1)/Personal

[TheOneRing]

We could use xattr and ntfs streams to "claim" a folder, if neither is supported for the folder we ignore it.
That way we should always be able to prevent multiple accounts from using the same default sync root.

[Salipa-Gurung]

Tested on :
ownCloud 6.0.0.13062-daily20240108 b46134
Libraries Qt 6.6.0, OpenSSL 3.1.4 24 Oct 2023
Using virtual files plugin: wincfapi
OS: windows-10.0.19045 (build arch: x86_64, CPU arch: x86_64)
QPA: windows

Server:
Infinite Scale 4.0.5 Community and ownCloud 10.13.4 (stable)

Adding account for brian after admin's account was added	Using default syncroot from the first account
Client provides different syncroot ✔️	Client prevents user from using same syncroot directory ✔️

Finish button remains clickable(does not perform any function) when error message is being displayed. ❔

cc: @erikjv