When the linux-pipewire plugin is unloaded, make sure to cleanup
the D-Bus proxy for the ScreenCast portal too. This effectively
doesn't change anything, but it's always good to keep up with the
code hygiene.
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.
Per upstream this function is not supposed to be called by users of
PipeWire, and will crash when called twice. Instead only call it on
recent versions of PipeWire where it is safe to call.
This also removes the nearly empty pipewire-common files.
