This PR enables QField to import a remote WebDAV folder into its imported projects' local storage. The functionality is available in the bottom right (+) import button within the local file picker section:

The PR is the foundational stone to implement simple download/upload synchronization once a folder is imported. That will come in a subsequent set of commits.

The PR also implements a download from and uploading to webdav server for imported webdav folders. The new pair of actions appear in the local file picker for any folder that is the root of an imported webdav folder, or any of its child folder:

When downloading/uploading against the root folder, all content will be pushed/pulled. If executed against a child folder, only that child folder will be considered. For both actions, files will be downloaded or uploaded whenever the last modified date does not match. It's a simple logic that is easy for users to understand.

A pair of download/upload actions has also been added into the (current) project folder's root view, provided the project resides within a webdav imported folder:

That makes for a nice UX where users don't need to drill down into a large list of imported folders.

Finally, when triggering a download or upload, QField will request a confirmation, informing users of data being overwritten:

@suricactus , I am wondering whether we should upgrade the QFieldCloudConnection class to also make use of the authentication manager to store / retrieve remembered passwords (see implementation here).

Edit: I should have mentioned that this work benefited from the pre-existing QtWebDAV library (this fork - https://github.com/PikachuHy/QtWebDAV - which itself was a fork of https://github.com/mhaller/qwebdavlib). We have taken the time to modernize its cmake configuration and add some extra functionality. Our fork lives here: https://github.com/m-kuhn/QtWebDAV