feat(book/cache): add export picture option

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

@@ -993,6 +993,7 @@
     <string name="bitmap_cache_size_summary">当前最大缓存 %1$s MB</string>
     <string name="bitmap_cache_size">图片绘制缓存</string>
     <string name="restart_to_apply_changes">重启应用更改</string>
-
+    <string name="export_pics_file">TXT导出图片</string>
     <!-- string end -->
+
 </resources>