If subfolder "icon" exists, then files named as "0","1","2"...
are used for icons in the bottom toolbar. file "0" to replace the left arrow button, "1" for right arrow button, "2" for the setting button,"3" for the home screen button,...
If a new button is defined (file name "20" exists), uweb also supports VectorDrawable with "android compiled XML" format. All icon files must be all VectorDrawables or 此时所有图标文件必须都是compiled XML格式，或都是普通的图像格式（png,jpg,gif等）。用户可将VectorDrawable XML文件放到安卓项目目录assets下面，重新生成apk文件以后将apk文件解压即可得compiled XML格式图标文件。
用户可定义任意多个按钮(100个按钮毫无压力)，其短按长按功能可在文件default.button及default.button2中定义，格式与文件default.link完全相同，每行从最后一个按钮开始重定义功能。
注意：刷新按钮及窗口按钮图标暂时不能重定义。