Sort by category in emoji picker (#4533)

Emojis are sorted by category within the emoji picker. The original
alphabetical sorting is preserved within categories.

This partially addresses #1868 and is only a small part of what is done
in #3300, but I think it is still enough to improve quality of life for
users.

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

@@ -26,5 +26,6 @@ data class Emoji(
     val shortcode: String,
     val url: String,
     @Json(name = "static_url") val staticUrl: String,
-    @Json(name = "visible_in_picker") val visibleInPicker: Boolean = true
+    @Json(name = "visible_in_picker") val visibleInPicker: Boolean = true,
+    val category: String?
 ) : Parcelable