Multiple workspace navigation

[jihwan]

it'll be very nice if I can group files under different names within one work space so I can do "find in group".
If it's too much, this will be good too -- opening several workspaces and
letting me navigate through them.

When I have 20-30 files opened at the same time, even though I do not
see all those files at the same time, when I try to pick one file I need among them, it's not easy to do.