This is the start of what will be a rather incisive surgery on
pipewire.c. Move a couple of functions to portal.c, since they're
not really related to the PipeWire code.
The portal.c file was introduced after the PipeWire code landed, and
handles acquiring a D-Bus connection to the portal interface, and also
has helpers to get properties from portals. The available cursor modes
property fits nicely in portal.c.
Move fetching the cursor mode to portal.c.
