Make an arbitrary directory root

[DenisaCG]

Added a root parameter to the authentication plugin - a variable containing the path to a directory within the bucket, which will act as a new root of the drive inside the FileBrowser. If left empty, the root will be considered as the default one (the directory holding the contents of the entire bucket).

When being set for the Drive object, the root is automatically formatted accordingly with the use of a helper function, which makes sure all leading and trailing / are eliminated and the path leads to an existing directory within the chosen bucket. If the path is faulty, theroot becomes the default one.

Updated the logic of all existing functionalities to consider the root when extracting the path, if it exists, and added a new root parameter in the dialog box for switching to another drive (if the parameter is left empty, the default one is considered).

[afshin]

Thank you!