Introduce document classes

[DigitalTimK]

Background

The current way to manage documents is very inconsistent (following the same inconsistent way as on the BBB-Server API side). Sometimes they are called documents and sometimes presentations. For example there is currently a class called "SomeThingDocument" but has a addPresentation-function.
In addition the addPresentation-function is unclear regarding their parameter and how they will support the required result. Either by URL or by embedded data stream. So a separation into two cases (=classes) makes senses.
On top the addPresentation-function required the user to pass the data-stream directly.

This PR will

• introduce DocumentUrl-class and DocumentFile-class (including their related abstract class and interface class)
• add related tests
• no passing of data-stream directly. Only the filename is required and this is done by the classes automatically.
• deprecate former presentation-related function and classes

This PR will close #244