Resets the paging3 changes of 3159 back to the (java) fragment code before.

Should be the basis for further not-so-rattling improvements.

app/src/main/java/com/keylesspalace/tusky/util/PairedList.kt

@@ -0,0 +1,74 @@
+package com.keylesspalace.tusky.util
+
+import androidx.arch.core.util.Function
+
+/**
+ * This list implementation can help to keep two lists in sync - like real models and view models.
+ *
+ * Every operation on the main list triggers update of the supplementary list (but not vice versa).
+ *
+ * This makes sure that the main list is always the source of truth.
+ *
+ * Main list is projected to the supplementary list by the passed mapper function.
+ *
+ * Paired list is newer actually exposed and clients are provided with `getPairedCopy()`,
+ * `getPairedItem()` and `setPairedItem()`. This prevents modifications of the
+ * supplementary list size so lists are always have the same length.
+ *
+ * This implementation will not try to recover from exceptional cases so lists may be out of sync
+ * after the exception.
+ *
+ * It is most useful with immutable data because we cannot track changes inside stored objects.
+ *
+ * @param T type of elements in the main list
+ * @param V type of elements in supplementary list
+ * @param mapper Function, which will be used to translate items from the main list to the
+ * supplementary one.
+ * @constructor
+ */
+class PairedList<T, V> (private val mapper: Function<T, out V>) : AbstractMutableList<T>() {
+    private val main: MutableList<T> = ArrayList()
+    private val synced: MutableList<V> = ArrayList()
+
+    val pairedCopy: List<V>
+        get() = ArrayList(synced)
+
+    fun getPairedItem(index: Int): V {
+        return synced[index]
+    }
+
+    fun getPairedItemOrNull(index: Int): V? {
+        return synced.getOrNull(index)
+    }
+
+    fun setPairedItem(index: Int, element: V) {
+        synced[index] = element
+    }
+
+    override fun get(index: Int): T {
+        return main[index]
+    }
+
+    override fun set(index: Int, element: T): T {
+        synced[index] = mapper.apply(element)
+        return main.set(index, element)
+    }
+
+    override fun add(element: T): Boolean {
+        synced.add(mapper.apply(element))
+        return main.add(element)
+    }
+
+    override fun add(index: Int, element: T) {
+        synced.add(index, mapper.apply(element))
+        main.add(index, element)
+    }
+
+    override fun removeAt(index: Int): T {
+        synced.removeAt(index)
+        return main.removeAt(index)
+    }
+
+    override val size: Int
+        get() = main.size
+}

app/src/main/java/com/keylesspalace/tusky/util/ViewDataUtils.kt

@@ -55,12 +55,13 @@ fun Status.toViewData(
     )
 }
 
+@JvmName("notificationToViewData")
 fun Notification.toViewData(
     isShowingContent: Boolean,
     isExpanded: Boolean,
     isCollapsed: Boolean
-): NotificationViewData {
-    return NotificationViewData(
+): NotificationViewData.Concrete {
+    return NotificationViewData.Concrete(
         this.type,
         this.id,
         this.account,