- Update python wrapper generation to genrate LinphoneWrapper.cs with a
   full documentation.
 - Update cmake to generate a html doc site with docfx for C# wrapper
 - minor fixes in lib documentation

Audio flowcontrol algorithm improved: try to eliminate silent audio buffers first, then use zero-crossing technique. This should cause sample drops to be more silent.
Also add properties to choose a drop policy (soft or basic), and set the silent threshold.

* Fix links to classes, enums and enumerators for every doc engines.
* Document “linphone” namespace in order top-level enums be indexed by Doxygen.
* Make @bctbx_list Doxygen macro to not generate code in HTML documentaiton.
* Fix several invalid reference in the C API documentation.

Added C/wrapped method to be able to add a generic content to a ChatMessage + added a custom content type multipart group chat test

Fixed displayed imdn sent when auto download incoming file transfer is enabled but chat message not marked as read

Reworked the callback algorithm on audio route change for more clarity, and add a fallback case for when the audio route does not match devices listed in linphone.

Fix a rare issue on iOS, where audio stops after permutting two calls. The AudioUnit re-use logic is modified so that re-use can only happen within the scope of a call, to be compatible with CallKit's policy.