Add option to prevent deleting bookmarked chapters (closes #2082)

app/src/main/java/eu/kanade/tachiyomi/ui/manga/MangaPresenter.kt

@@ -494,8 +494,14 @@ class MangaPresenter(
      * @param chapters the chapters to delete.
      */
     private fun deleteChaptersInternal(chapters: List<ChapterItem>) {
-        downloadManager.deleteChapters(chapters, manga, source)
-        chapters.forEach {
+        val filteredChapters = if (!preferences.removeBookmarkedChapters()) {
+            chapters.filterNot { it.bookmark }
+        } else {
+            chapters
+        }
+
+        downloadManager.deleteChapters(filteredChapters, manga, source)
+        filteredChapters.forEach {
             it.status = Download.NOT_DOWNLOADED
             it.download = null
         }

app/src/main/java/eu/kanade/tachiyomi/ui/setting/SettingsDownloadController.kt

@@ -84,6 +84,11 @@ class SettingsDownloadController : SettingsController() {
                 defaultValue = "-1"
                 summary = "%s"
             }
+            switchPreference {
+                key = Keys.removeBookmarkedChapters
+                titleRes = R.string.pref_remove_bookmarked_chapters
+                defaultValue = false
+            }
         }
 
         val dbCategories = db.getCategories().executeAsBlocking()