Allow renaming of Nextcloud favorites in Nautilus sidebar

**Description:**

Currently, the Nextcloud Desktop Client integrates with Nautilus (GNOME Files) by exposing synchronized folders as favorites in the sidebar. These entries are automatically named based on the local folder names.

However, it is not possible to rename these favorites independently from the underlying folder structure. This leads to limitations in usability, especially for users who:

* sync multiple Nextcloud instances or accounts
* use nested or technically named directories
* want to present a cleaner, user-friendly navigation structure in Nautilus

**Expected behavior:**

Users should be able to assign custom display names to Nextcloud favorites in the Nautilus sidebar, without affecting:

* the actual folder name on disk
* the synchronization logic
* the remote folder structure on the Nextcloud server

**Example use case:**

Assume two Nextcloud folders like:

`~/Nextcloud/MyPrivateCloud`
`~/Nextcloud/MyOrganizationCloud`

Today, due to the max length of favorites names, they are shown in Nautilus sidebar like this:

`Nextcloud/...teCloud`
`Nextcloud... ationCloud`

They could be displayed in a beautified manner in Nautilus sidebar simply as:

`MyPrivateCloud`
`MyOrganizationCloud`

Or even like this:

`My files`
`Job`

**Proposed approaches (optional):**

* Store a local mapping (e.g. via metadata or config file) between folder path and display name
* Integrate with Nautilus bookmark system (which already supports custom names)
* Provide this feature via the Nextcloud client UI (right-click → “Rename favorite”)

**Why this matters:**

This would improve usability and navigation clarity for users with nested folder structures, without introducing any risk to data integrity or sync behavior.

**Environment:**

* OS: Linux (Arch Linux, GNOME 50)
* File manager: Nautilus

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Happy to provide additional details or test implementations.