feature: the session facility to save opened buffers.

[sakurawald]

When quit the lem editor, and restart it.
We will need to re-open the opened buffers one by one, and tweak the window layout again and again.

If lem can save current working set, and restore it, and it will makes the work-flow more smooth. (Imaging just open your project in a new day with lem editor, and all buffers are opened and window layout is restored, just as yesterday you quit it.)