添加主窗口关于菜单和刷新封面功能

- 主窗口添加工具栏关于应用菜单
- AboutActivity使用AppBarLayout与主窗口一致
- 添加返回按钮到AboutActivity工具栏
- 去掉主窗口刷新菜单
- onResume时刷新相册集封面

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

@@ -4,6 +4,7 @@
     <string name="refresh">Refresh</string>
     <string name="settings">Settings</string>
     <string name="settings_title">Settings</string>
+    <string name="about">About</string>
     <string name="folder_path">Folder Path</string>
     <string name="enter_folder_path">Enter folder path</string>
     <string name="save">Save</string>