ui: Simplify interaction with mail dir watcher

Reduce the message cmd and just use a call back to pass back the scanned
mails.

Signed-off-by: Fam Zheng <famz@redhat.com>
ui: Refactor to Views

Signed-off-by: Fam Zheng <famz@redhat.com>
ui: Fix refresh window after start

Signed-off-by: Fam Zheng <famz@redhat.com>
ui: Show mail content by pressing enter

Scroll with "j", "k", "d", "u".

Signed-off-by: Fam Zheng <famz@redhat.com>