Don't accidentally hide relevant error banners

We use error banners for intermittent errors (failing to join a room),
along with more long-standing/important errors (like going offline.) And
due to the nature of how we manage banner visibility - for example, the
online check could accidentally hide joining rooms errors which is
pretty bad.

To fix this, each error banner is assigned an id. All errors can still
overwrite each other as they could before, but they can only hide
banners of their own kind.

src/app/qml/RoomPage.qml

@@ -79,9 +79,9 @@ Kirigami.Page {
 
         if (root.currentRoom.tagNames.includes("m.server_notice")) {
             banner.text = i18nc("@info", "This room contains official messages from your homeserver.")
-            banner.visible = true;
+            banner.show("message");
         } else {
-            banner.visible = false;
+            banner.hideIf("message");
         }
     }
 
@@ -90,10 +90,10 @@ Kirigami.Page {
         function onIsOnlineChanged() {
             if (!root.currentRoom.connection.isOnline) {
                 banner.text = i18nc("@info:status", "NeoChat is offline. Please check your network connection.");
-                banner.visible = true;
                 banner.type = Kirigami.MessageType.Error;
+                banner.show("offline");
             } else {
-                banner.visible = false;
+                banner.hideIf("offline");
             }
         }
     }
@@ -101,9 +101,23 @@ Kirigami.Page {
     header: Kirigami.InlineMessage {
         id: banner
 
+        // Used to keep track of messages so we can hide the right one at the right time
+        property string messageId
+
         showCloseButton: true
         visible: false
         position: Kirigami.InlineMessage.Position.Header
+
+        function show(msgid: string): void {
+            messageId = msgid;
+            visible = true;
+        }
+
+        function hideIf(msgid: string): void {
+            if (messageId == msgid) {
+                visible = false;
+            }
+        }
     }
 
     Loader {
@@ -204,7 +218,7 @@ Kirigami.Page {
         function onShowMessage(messageType, message) {
             banner.text = message;
             banner.type = messageType;
-            banner.visible = true;
+            banner.show("generic");
         }
 
         function onShowEventSource(eventId) {