decklink: Duplex-Mode Property for DeckLink Duo2 and Quad2

This PR adds support for the Duplex-Mode Condifiguration of DeckLink Duo2 and Quad2 cards to the decklink-Elements in form of a duplex-mode-Property with two enum-values: half and full.

Without setting the Duplex-Mode, the two aforementioned cards will self-configure into Full-Duplex mode. In Full-Duplex-Mode they can perform Alpha-Channel Keying in Playout mode.

This PR allows setting the Duplex-Mode of these cards to Half-Duplex, in which each of the Connectors acts as a stand-alone Input or Output.

The PR also adds extensive documentation on how to configure the different modes with example pipeliens by means of GtkDoc documentation in the respective elements.

I'm unsure about is which mode should be default. The DeckLink cards themselfs operate by default in Full-Duplex mode, but this mode requires specific input with alpha-channel video to be useful and is only really useful in playout-mode. Therefor I opted to make half-duplex mode the default, because it let's the Duo2 and Quad2 cards behave like the other DeckLink models and in an more expected way. Because up until this PR there was no way to set the duplex-mode property, this is a breaking change for users who relied upon the default full-duplex mode. I'm unsure how the GStreamer Project deals with such breaking changes and am open for suggestions.