Coordinator stops (#207)

* Add `stop` method to `Coordinator`

* Use new stop method to cleanup room screen

* Call stop implicitly when removing a child coordinator

* Revert placeholder avatar font change

* Fix PR remarks

ElementX/Sources/Screens/RoomScreen/View/RoomScreen.swift

@@ -41,9 +41,6 @@ struct RoomScreen: View {
             }
         }
         .alert(item: $context.alertInfo) { $0.alert }
-        .onDisappear {
-            context.send(viewAction: .viewDisappeared)
-        }
     }
     
     private func sendMessage() {

ElementX/Sources/Screens/RoomScreen/View/Timeline/PlaceholderAvatarImage.swift

@@ -26,7 +26,8 @@ struct PlaceholderAvatarImage: View {
             Text(textForImage)
                 .padding(4)
                 .foregroundColor(.white)
-                .font(.title2.bold())
+                .font(.system(size: 200).weight(.semibold))
+                .minimumScaleFactor(0.001)
         }
         .aspectRatio(1, contentMode: .fill)
     }