Make offline presence to stop sync

Setting the presence of an account to offline
will make the client to end sync task and will
prevent messages from being sent.

Setting it to online again or any other presence
will start sync task again.

Left:
- Local echo to presence change
- UI Control to affect all members presence
- Block more requests to be sent (e.g. member
  actions)

src/backend/backend.py

@@ -132,7 +132,7 @@ class Backend:
         )
 
         try:
-            await client.login(password)
+            await client.login(password, order=order)
         except MatrixError:
             await client.close()
             raise
@@ -142,22 +142,7 @@ class Backend:
             await client.logout()
             return client.user_id
 
-        if order is None and not self.models["accounts"]:
-            order = 0
-        elif order is None:
-            order = max(
-                account.order
-                for i, account in enumerate(self.models["accounts"].values())
-            ) + 1
-
-        account = Account(client.user_id, order)
-
-        self.clients[client.user_id]            = client
-        self.models["accounts"][client.user_id] = account
-
-        # Get or create presence for account
-        presence = self.presences.setdefault(client.user_id, Presence())
-        presence.members["account", client.user_id] = account
+        self.clients[client.user_id] = client
 
         return client.user_id
 
@@ -168,7 +153,6 @@ class Backend:
         token:      str,
         device_id:  str,
         homeserver: str = "https://matrix.org",
-        order:      int = -1,
         state:      str = "online",
     ) -> None:
         """Create and register a `MatrixClient` with known account details."""
@@ -178,14 +162,7 @@ class Backend:
             user=user_id, homeserver=homeserver, device_id=device_id,
         )
 
-        account = Account(user_id, order)
-
-        self.clients[user_id]            = client
-        self.models["accounts"][user_id] = account
-
-        # Get or create presence for account
-        presence = self.presences.setdefault(user_id, Presence())
-        presence.members["account", user_id] = account
+        self.clients[user_id] = client
 
         await client.resume(user_id, token, device_id, state)
 
@@ -194,14 +171,19 @@ class Backend:
         """Call `resume_client` for all saved accounts in user config."""
 
         async def resume(user_id: str, info: Dict[str, Any]) -> str:
+            # Get or create account model
+            self.models["accounts"].setdefault(
+                user_id, Account(user_id, info.get("order", -1)),
+            )
+
             await self.resume_client(
                 user_id    = user_id,
                 token      = info["token"],
                 device_id  = info["device_id"],
                 homeserver = info["homeserver"],
-                order      = info.get("order", -1),
                 state      = info.get("presence", "online"),
             )
+
             return user_id
 
         return await asyncio.gather(*(