Add inter* GStreamer elements to build
libopenwebrtc.exp: Remove obsolete symbol
transport: Fix refcounting of closures and make element types of the candidate lists explicit
transport: Fix closure refcounting in the media session
transport: Fix up refcounting and locking for the payloads and source of the media session

As a side effect we can now also easily remove/replace a send payload.
transport: Take ownership of added media sessions in the transport agent
transport: Catch errors in new send source closure in the transport agent
transport: Add some more assertions
transport: Make sure the GResolver callback is called from our main context

Also unref the default resolver in the right place.
owr: Make sure that declarations are always before non-static function definitions

And fix the existing inconsistencies that are there already.

Also remove _owr_get_main_loop() as it's not used anywhere.
local: Fix some set-but-never-used variable compiler warnings
transport: Fix various small memory leaks in the transport agent