Improve notifications fetching (#1930)

* Improve notifications fetching

 - Only fetch notifications up to the latest fetched one
 - Use timeline markers to avoid showing already seen notifications

* Apply some of the suggestions

app/src/main/java/com/keylesspalace/tusky/entity/Marker.kt

@@ -0,0 +1,15 @@
+package com.keylesspalace.tusky.entity
+
+import com.google.gson.annotations.SerializedName
+import java.util.*
+
+/**
+ * API type for saving the scroll position of a timeline.
+ */
+data class Marker(
+        @SerializedName("last_read_id")
+        val lastReadId: String,
+        val version: Int,
+        @SerializedName("updated_at")
+        val updatedAt: Date
+)