Create updated alert window

One function that would be useful is to have a better alert popup. Right now for alerts we use the "alert" function in javascript. But it is ugly.

If we could have somewhere a

LMalert(message)

then we could post messages like:

"please enter an object name"

"This polygon was entered by another user. You can only modify polygons that you have entered. Do not forget to sign in if you want to be able to edit your annotations"

and so on

首先定位现有的 JavaScript alert 调用，以及显示用户消息的入口点。根据 issue 中的示例定义可复用的 LMalert 行为，然后验证新的弹窗会呈现这些消息，并替换相关的原生 alert。

由索引模型根据 Issue 内容生成。