feat(ui+ffi): Implement Room::id as a shortcut.

It's possible to do `room.inner_room().room_id()` but it's just simpler
to call `room.id()`. This patch adds this shortcut.

It's especially useful for FFI users, where creating a “full
room” (`room_item.full_room().room_id()`) may be expensive.

bindings/matrix-sdk-ffi/src/room_list.rs

@@ -194,6 +194,10 @@ pub struct RoomListItem {
 
 #[uniffi::export]
 impl RoomListItem {
+    fn id(&self) -> String {
+        self.inner.id().to_string()
+    }
+
     fn name(&self) -> Option<String> {
         RUNTIME.block_on(async { self.inner.name().await })
     }

crates/matrix-sdk-ui/src/room_list/room.rs

@@ -4,7 +4,7 @@ use std::sync::Arc;
 
 use async_once_cell::OnceCell as AsyncOnceCell;
 use matrix_sdk::{SlidingSync, SlidingSyncRoom};
-use ruma::api::client::sync::sync_events::v4::RoomSubscription;
+use ruma::{api::client::sync::sync_events::v4::RoomSubscription, RoomId};
 
 use super::Error;
 use crate::{timeline::EventTimelineItem, Timeline};
@@ -58,6 +58,11 @@ impl Room {
         })
     }
 
+    /// Get the room ID.
+    pub fn id(&self) -> &RoomId {
+        self.inner.room.room_id()
+    }
+
     /// Get the best possible name for the room.
     ///
     /// If the sliding sync room has received a name from the server, then use