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

Start by locating the existing JavaScript alert calls and the entry points that display user messages. Define the reusable LMalert behavior from the examples in the issue, then verify that the new popup presents those messages and replaces the relevant native alerts.

索引モデルが issue の本文から書いたものです。