Sent in chans and subscriptions columns

Now it shows "from" instead of "to" column when selecting sent folder in
subscriptions or chans.

Fixes #53

Messages tab resizable

- the separator between folder tree and message list is now resizable
- the separator between list of messages and message content is
resizable
- only for message tab, chans, subscriptions and others are still not
resizable

Fix: do not update account name from folder name

Last commit caused changes in unread count of folder to change the
account name. This is now fixed.

Email gateway integration

Sending/receiving from the send tab, reply from inbox and
registration/unregistration context menu.

'v0.4.4'

'v0.4.3'