Make the authentication easier to customize

With NXDRIVE-1291, the user's browser will be used for the authentication.
It will redirect to a custom protocol URL.

The custom protocol URL will work only when Drive is packaged, so developers will be stuck. As the WebEngine module is removed from Windows packages only (and our devs are on GNU/Linux and macOS), we imagined that workaround:

• move the connection code that uses WebEngine from gui/*.py to gui/dev/xxx.py
• set the envar NXDRIVE_DEV to 1
• in gui/application.py, check the envar and if defined, import the dev package and use its class instead of the "legacy" one
• in ndrive.spec, exclude the nxdrive.gui.dev package