don't transfer portal stuff

as it needs dbus to work, and we don't forward dbus

discard the 'extra' ones early, it may help prevent some applications requesting them

also add some docstrings

xpra/clipboard/core.py

@@ -65,6 +65,7 @@ def get_discard_targets(envname: str = "DISCARD", default_value: Iterable[str] =
         r"^dyn\.",
         r"^resource-transfer-format",  # eclipse
         r"^x-special/",  # ie: gnome file copy
+        r"^application/vnd.portal.",  # portal uses dbus, which is not forwarded
     )
 ))
 # targets some applications are known to request,
@@ -103,7 +104,7 @@ def must_discard_extra(target: str) -> bool:
 
 def _filter_targets(targets: Iterable[str]) -> Sequence[str]:
     targets_strs = tuple(bytestostr(x) for x in targets)
-    f = tuple(target for target in targets_strs if not must_discard(target))
+    f = tuple(target for target in targets_strs if not (must_discard(target) or must_discard_extra(target)))
     log("_filter_targets(%s)=%s", csv(targets_strs), f)
     return f
 
@@ -529,9 +530,11 @@ def _process_clipboard_token(self, packet: PacketType) -> None:
         proxy.got_token(targets, target_data, claim, synchronous_client)
 
     def local_targets(self, remote_targets: Iterable[str]) -> Sequence[str]:
+        """ filter remote targets to values that can be used locally """
         return _filter_targets(remote_targets)
 
     def remote_targets(self, local_targets: Iterable[str]) -> Sequence[str]:
+        """ export local targets """
         return _filter_targets(local_targets)
 
     def _munge_raw_selection_to_wire(self, target: str, dtype: str, dformat: int, data) -> tuple[Any, Any]: