introspection: Call g-ir-scanner correctly

This provides a symbol prefix separate from the identifier prefix,
corrects the identifier prefix to not include the trailing underscore,
specifies the exported pkg-config file, and provides a list of C
headers.

https://bugs.freedesktop.org/show_bug.cgi?id=86589
Add ICE-TCP candidates

Also to the nice transmitter
nice: Add support for reliable nice agent and selection of ice-udp/ice-tcp
rtpsession: Add support for send-rtcp-mux on fs-rtp-session and nice transmitter
fs-gui: Remove duplicated FsUIParticipant
fs-gui: Remove implicit __del__() with explicit destroy() calls
nice-transmitter: Fix off-by-one

In fs_nice_transmitter_set_send_component_mux(), the component IDs,
which start from 1, are used as nicesinks array indexes and
nicesinks[0] is always NULL.
Revert "nice stream transmitter: Ignore ICE-TCP candidates for now"

This reverts commit 137e5607083f1b7522849c3e1f5976736ca0d098.