feat: webdav书籍排序

添加 按更新时间排序，按文件名排序的选项。
默认排序规则为按文件更新时间倒序排列

app/src/main/res/values/strings.xml

@@ -352,6 +352,7 @@
     <string name="sort">Sort</string>
     <string name="sort_auto">Sort automatically</string>
     <string name="sort_manual">Sort manually</string>
+    <string name="sort_default">Sort default</string>
     <string name="sort_by_name">Sort by name</string>
     <string name="go_to_top">Scroll to the top</string>
     <string name="go_to_bottom">Scroll to the bottom</string>