Time panel now always talks about "events" instead of "messages" (#2247)

Tiny follow-up to #2233

Under the hood this is nuanced and we don't always get the names right:
An Event is a set of component changes for a single given entity at a
single given timepoint. A Message on the other hand is something we send
from api to store and may contain many of these Events and potentially
other things.

That said, I didn't find other occurrences where this is clearly labeled
incorrectly, but there gotta be more!

### What

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)

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PR Build Summary: https://build.rerun.io/pr/2247

crates/re_viewer/src/ui/time_panel/data_density_graph.rs

@@ -523,17 +523,17 @@ fn show_row_ids_tooltip(
     egui_ctx: &egui::Context,
     item: &Item,
     time_range: TimeRange,
-    num_messages: usize,
+    num_events: usize,
 ) {
-    if num_messages == 0 {
+    if num_events == 0 {
         return;
     }
 
     egui::show_tooltip_at_pointer(egui_ctx, egui::Id::new("data_tooltip"), |ui| {
-        if num_messages == 1 {
-            ui.label(format!("{num_messages} message"));
+        if num_events == 1 {
+            ui.label(format!("{num_events} event"));
         } else {
-            ui.label(format!("{num_messages} messages"));
+            ui.label(format!("{num_events} events"));
         }
 
         ui.add_space(8.0);