Fetching, sending, etc should not lock all wnidows

[gullevek]

When I open a new mail folder, send mail or do anything, all other windows are "locked" and do not allow any actions.

If I open a large folder with a lot of mails, I always have to wait until the fetching of messages is complete. This is rather annoying.

Such an operation should run in the background and other operations should still be possible.