This pr removes the backup db (sync db) of the cloud syncing strategy when a different server location is used. This is to avoid the situation when later incoming db is not based on previously uploaded sync db.
This commit adds cross-device sync ability for two plugins: reading statistics and vocabulary builder. It relies on user setting up a Cloud server (DropBox and WebDAV but not FTP though) and designating a path. Behind the curtains sqlite databases are being passed around and updated.
UI-wise, for the statistics plugin, two new menu items Synchronize now and Cloud sync to set it up (might not be the best wording) are added. As for vocabulary builder, a similar Cloud sync button is added to the menu and a shortcut icon button to Synchronize now is pinned at the bottom corner.
CloudStorage new features: WebDAV creating folders and uploading files. And a new widget-like sync server chooser. In the end I decided not to add automatic sync, as the SQL commands part seem a bit much.
