Sorry this is such a massive commit. The entangled nature of the
code in pipewire.c makes it impossible to reasonably split this
in smaller commits.
Move all D-Bus / portals code from pipewire.c into the recently
introduced screencast-portal.c.
As we slowly split portal code from PipeWire code, what is currently
called pipewire-capture will become the screencast portal specific
code.
Preemptively rename the corresponding files and functions. Next commits
will continue the excision of portal code from pipewire.c.
