feat(dnd_destination): xdg file transfer portal support

Requires the `xdg-portal` feature to be enabled to use these features.

- Adds `DndDestination::on_file_transfer` method to handle `application/vnd.portal.filetransfer` drop requests
- Adds `command::file_transfer_receive` function to handle the file transfer request messages
- Adds `command::file_transfer_send` to initiate a file transfer from the application

src/widget/dnd_destination.rs

@@ -40,6 +40,8 @@ pub fn dnd_destination_for_data<'a, T: AllowedMimeTypes, Message: 'static>(
 
 static DRAG_ID_COUNTER: AtomicU64 = AtomicU64::new(0);
 const DND_DEST_LOG_TARGET: &str = "libcosmic::widget::dnd_destination";
+#[cfg(feature = "xdg-portal")]
+pub const FILE_TRANSFER_MIME: &str = "application/vnd.portal.filetransfer";
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct DragId(pub u128);
@@ -73,6 +75,8 @@ pub struct DndDestination<'a, Message> {
     on_action_selected: Option<Box<dyn Fn(DndAction) -> Message>>,
     on_data_received: Option<Box<dyn Fn(String, Vec<u8>) -> Message>>,
     on_finish: Option<Box<dyn Fn(String, Vec<u8>, DndAction, f64, f64) -> Message>>,
+    #[cfg(feature = "xdg-portal")]
+    on_file_transfer: Option<Box<dyn Fn(String) -> Message>>,
 }
 
 impl<'a, Message: 'static> DndDestination<'a, Message> {
@@ -99,6 +103,8 @@ impl<'a, Message: 'static> DndDestination<'a, Message> {
             on_action_selected: None,
             on_data_received: None,
             on_finish: None,
+            #[cfg(feature = "xdg-portal")]
+            on_file_transfer: None,
         }
     }
 
@@ -124,6 +130,8 @@ impl<'a, Message: 'static> DndDestination<'a, Message> {
             on_finish: Some(Box::new(move |mime, data, action, _, _| {
                 on_finish(T::try_from((data, mime)).ok(), action)
             })),
+            #[cfg(feature = "xdg-portal")]
+            on_file_transfer: None,
         }
     }
 
@@ -159,6 +167,8 @@ impl<'a, Message: 'static> DndDestination<'a, Message> {
             on_action_selected: None,
             on_data_received: None,
             on_finish: None,
+            #[cfg(feature = "xdg-portal")]
+            on_file_transfer: None,
         }
     }
 
@@ -237,6 +247,20 @@ impl<'a, Message: 'static> DndDestination<'a, Message> {
         self
     }
 
+    /// Add a message that will be emitted instead of [`on_data_received`](Self::on_data_received) if the dropped files
+    /// are offered through the xdg share portal. You can then use [`crate::command::file_transfer_receive`]
+    /// with the key to receive the files.
+    #[cfg(feature = "xdg-portal")]
+    #[must_use]
+    pub fn on_file_transfer(mut self, f: impl Fn(String) -> Message + 'static) -> Self {
+        match self.mime_types.iter().position(|v| v == "text/uri-list") {
+            Some(i) => self.mime_types.insert(i, Cow::Borrowed(FILE_TRANSFER_MIME)),
+            None => self.mime_types.push(Cow::Borrowed(FILE_TRANSFER_MIME)),
+        }
+        self.on_file_transfer = Some(Box::new(f));
+        self
+    }
+
     /// Returns the drag id of the destination.
     ///
     /// # Panics
@@ -496,6 +520,13 @@ impl<Message: 'static> Widget<Message, crate::Theme, crate::Renderer>
                     "offer data id={my_id:?} mime={mime_type:?} bytes={}",
                     data.len()
                 );
+
+                #[cfg(feature = "xdg-portal")]
+                if mime_type == FILE_TRANSFER_MIME && let Some(f) = self.on_file_transfer.as_ref() && let Ok(s) = String::from_utf8(data[..data.len() - 1].to_vec()) {
+                    shell.publish(f(s));
+                    return event::Status::Captured;
+                }
+
                 if let (Some(msg), ret) = state.on_data_received(
                     mime_type,
                     data,