Support for DropBox

OpenTodoList aims to allow users to keep in control of their own data. This also meant that initially we focused purely on WebDAV as sync protocol to keep multiple devices in sync. This is still a good and right decision (and won't go away).

However, for a lot of users, setting up their own server to enable sync of data, is not possible.

Hence, it would be cool if the app would support other services out of the box as well. The probably best known (and most widely used) such service is DropBox. DropBox has a well documented API which can be used to query file information, upload as well as download files. Hence, it perfectly fits in with the refactored sync code (which meanwhile lives in its own project).

Update

Introducing DropBox support requires also some larger changes in the Account handling within the app. Hence, here's a complete list of points that need to be (manually) tested before merging:

• Creating a new WebDAV account.
• Edit WebDAV account.
• Create a new ownCloud account.
• Edit ownCloud Account.
• Create a new NextCloud account.
• Edit NextCloud account.
• Create a new DropBox account.
• Edit DropBox account.
• Generation of new Access Tokens for DropBox.
• Test WebDAVAccount::login() in unit test.