设置里增加书籍文件夹配置,方便切换外部书籍保存文件夹

app/src/main/res/xml/pref_config_other.xml

@@ -56,6 +56,11 @@
             android:key="userAgent"
             android:title="UserAgent" />
 
+        <io.legado.app.ui.widget.prefs.Preference
+            android:key="defaultBookTreeUri"
+            android:title="@string/book_tree_uri_t"
+            android:summary="@string/book_tree_uri_s" />
+
         <io.legado.app.ui.widget.prefs.Preference
             android:key="uploadRule"
             android:title="@string/direct_link_upload_rule"