refactor(ui): Add ObservableItemsTransaction::has_local.

This patch implements the `has_local` method on
`ObservableItemsTransaction`, which is way faster than the previous the
previous solution which was to iterate over all items to find at least
one local timeline item.

crates/matrix-sdk-ui/src/timeline/controller/observable_items.rs

@@ -320,11 +320,18 @@ impl<'observable_items> ObservableItemsTransaction<'observable_items> {
         self.push_back(timeline_item, None);
     }
 
+    /// Check whether there is at least one [`Local`] timeline item.
+    ///
+    /// [`Local`]: super::EventTimelineItemKind::Local
+    pub fn has_local(&self) -> bool {
+        matches!(self.items.last(), Some(timeline_item) if timeline_item.is_local_echo())
+    }
+
     /// Push a new [`TimelineStart`] virtual timeline item.
     ///
     /// # Invariant
     ///
-    /// A [`TimelineStart`] is always the first item if present..
+    /// A [`TimelineStart`] is always the first item if present.
     ///
     /// # Panics
     ///

crates/matrix-sdk-ui/src/timeline/controller/state_transaction.rs

@@ -476,14 +476,12 @@ impl<'a> TimelineStateTransaction<'a> {
     }
 
     pub(super) fn clear(&mut self) {
-        let has_local_echoes = self.items.iter().any(|item| item.is_local_echo());
-
         // By first checking if there are any local echoes first, we do a bit
         // more work in case some are found, but it should be worth it because
         // there will often not be any, and only emitting a single
         // `VectorDiff::Clear` should be much more efficient to process for
         // subscribers.
-        if has_local_echoes {
+        if self.items.has_local() {
             // Remove all remote events and virtual items that aren't date dividers.
             self.items.for_each(|entry| {
                 if entry.is_remote_event()