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.
This is very much like previous commit, but there's a catch: there
already was an enumeration in place, which is replaced in this
commit. The obs_pw_capture_type enum was introduced before splitting
the portal code into a separate file, and the enum itself is specific
to the screencast portal, so the appropriate place to enumerate it
is in portal.h.
For completude, PORTAL_CAPTURE_TYPE_VIRTUAL was added to the enum,
even though we never used, and probably never will.
The values are still the same, since both the old and this new enum
were extracted from the screencast portal [1].
https://github.com/flatpak/xdg-desktop-portal/blob/main/data/org.freedesktop.portal.ScreenCast.xml#L290-300
