Correctly handle initial sync member events

Previously, member events in initial syncs (unless user had "hide member
events" and "hide profile events" set to false in their config) were
completely discarded with the help of a sync filter, instead of simply
being hidden like events loaded from room backfilling.

This was done due to the common case of rooms getting only
userconfig-ignored/hidden events on startup (especially with the
low number of initial events requested for lazy initial sync),
and thus having nothing to show as "last message" in
the room list (room delegate subtitles).

Other problems resulted from this, like missing join/leave events
when the config was set to hide profile events but not other
member events, and the "Members not synced" (#54) in encrypted room
with recent discarded member events.

The discarding filter is no longer used. Instead, if a room in the room
list has no visible "last message" and is currently visible to the user,
messages will be lazily fetched until we find something adequate or the
room goes out of view.

src/gui/MainPane/RoomDelegate.qml

@@ -6,10 +6,14 @@ import Clipboard 0.1
 import ".."
 import "../Base"
 import "../Base/HTile"
+import "../PythonBridge"
 
 HTile {
     id: room
 
+    property Future loadEventsFuture: null
+    property bool moreToLoad: true
+
     readonly property bool joined: ! invited && ! parted
     readonly property bool invited: model.inviter_id && ! parted
     readonly property bool parted: model.left
@@ -195,4 +199,29 @@ HTile {
             })
         }
     }
+
+    Component.onDestruction: if (loadEventsFuture) loadEventsFuture.cancel()
+
+    Timer {
+        running:
+            ! accountModel.connecting &&
+            accountModel.presence !== "offline" &&
+            ! lastEvent &&
+            moreToLoad
+
+        interval: 1000
+        triggeredOnStart: true
+        onTriggered: if (! loadEventsFuture) {
+            loadEventsFuture = py.callClientCoro(
+                model.for_account,
+                "load_past_events",
+                [model.id],
+                more => {
+                    if (! room) return  // delegate was destroyed
+                    loadEventsFuture = null
+                    moreToLoad       = more
+                }
+            )
+        }
+    }
 }