Decide which panel should be active for a dock based on ordering panels (#22346)

This means that `workspace::ToggleRightDock` will open the assistant if
no right-dock panel has been manually activated, instead of the chat as
before. Also cleans up the `active_panel_index` logic a bit.

cc @nathansobo 

Release Notes:

- Make `workspace::ToggleRightDock` open the assistant panel if no
right-dock panel has yet been activated

crates/collab_ui/src/chat_panel.rs

@@ -1141,6 +1141,7 @@ impl Panel for ChatPanel {
             ChatPanelButton::WhenInCall => ActiveCall::global(cx)
                 .read(cx)
                 .room()
+                .filter(|room| room.read(cx).contains_guests())
                 .map(|_| ui::IconName::MessageBubbles),
         }
     }
@@ -1159,6 +1160,10 @@ impl Panel for ChatPanel {
             .room()
             .is_some_and(|room| room.read(cx).contains_guests())
     }
+
+    fn activation_priority(&self) -> u32 {
+        7
+    }
 }
 
 impl EventEmitter<PanelEvent> for ChatPanel {}

crates/collab_ui/src/collab_panel.rs

@@ -2763,6 +2763,10 @@ impl Panel for CollabPanel {
     fn persistent_name() -> &'static str {
         "CollabPanel"
     }
+
+    fn activation_priority(&self) -> u32 {
+        6
+    }
 }
 
 impl FocusableView for CollabPanel {

crates/collab_ui/src/notification_panel.rs

@@ -731,6 +731,10 @@ impl Panel for NotificationPanel {
     fn toggle_action(&self) -> Box<dyn gpui::Action> {
         Box::new(ToggleFocus)
     }
+
+    fn activation_priority(&self) -> u32 {
+        8
+    }
 }
 
 pub struct NotificationToast {