Update ranking (#1772)

* Add LibraryUpdateRanker

This class provides various functions to generate Comparators that can 
be used to order the manga to update.

One such ordering is by relevance:
It prioritises manga that were updated more recently.

Another Ordering is by lexicographic order:
This is the default behaviour.

* Use relevanceRanking scheme

Instead of default(noRanking/lex ranking) now mangaList is sorted with 
relevanceRanking.

* Add UI and associated variables & strings for Update Ranking.

* Use user preferences to determine update ranking scheme.

* Refactor relevanceRanking to latestFirstranking.

This name seems to better reflect the ranking scheme and frees up the 
name relevanceRanking for future use.

* Set latestFirst scheme as default.

(Changing over from lexicographic scheme)

* Fix 1

[Convert LibraryUpdateRanker to a object.](./files/82f263749f0ae775385b23dd919f1865360db969#r287513539)

[Nitpick: Add lines](./files/82f263749f0ae775385b23dd919f1865360db969#r287540256)

[Replace Java comparator](./files/82f263749f0ae775385b23dd919f1865360db969#r287539976)

[Nitpick: Add local variable](./files/82f263749f0ae775385b23dd919f1865360db969#r287514805)

* Fix 2

[Weird import](./files/82f263749f0ae775385b23dd919f1865360db969#r287513709)

[Default value](./files/82f263749f0ae775385b23dd919f1865360db969#r287540064)

[Use existing Strings](./files/82f263749f0ae775385b23dd919f1865360db969#r287514476)

[Use Library update order](./files/82f263749f0ae775385b23dd919f1865360db969#r287540204)

app/src/main/java/eu/kanade/tachiyomi/data/preference/PreferenceKeys.kt

@@ -87,6 +87,8 @@ object PreferenceKeys {
 
     const val libraryUpdateCategories = "library_update_categories"
 
+    const val libraryUpdatePrioritization = "library_update_prioritization"
+
     const val filterDownloaded = "pref_filter_downloaded_key"
 
     const val filterUnread = "pref_filter_unread_key"

app/src/main/java/eu/kanade/tachiyomi/data/preference/PreferencesHelper.kt

@@ -143,6 +143,8 @@ class PreferencesHelper(val context: Context) {
 
     fun libraryUpdateCategories() = rxPrefs.getStringSet(Keys.libraryUpdateCategories, emptySet())
 
+    fun libraryUpdatePrioritization() = rxPrefs.getInteger(Keys.libraryUpdatePrioritization, 1)
+
     fun libraryAsList() = rxPrefs.getBoolean(Keys.libraryAsList, false)
 
     fun downloadBadge() = rxPrefs.getBoolean(Keys.downloadBadge, false)