Let buffers be enable to be manually sorted

[simnalamburt]

You should be able to change the order of channels as you like.

[heejongahn]

Title

Enable manual sorting of the buffers

Body

For now, prototype front-end is using elm's Dict for saving the buffers(bufferMap). As this is ordered map, we cannot manually change the order within the buffers. However, there has to be a way to do that, because users will want to configure the order of the buffers based on their preferences.

We can start from giving a unique id to each buffer (as a record member) and sort the buffers based on their id.