Ubuntu: Fix password dialog

Password dialog now uses a composer sheet, which also displays any
authentication errors
raised.

When launching app, if an imap account already exists then we skip the
settings page and
go straight to mailbox view where the password dialog will open
requesting password.

If an authentication error is raised we show an additional message that
explains to
either retry entering password or click cancel to return to the settings
page. We do not want the
user to be able to view mailbox without a connection. So this message
tries to make it clear what
clicking Cancel will do

REVIEW: 117380

Changed dialog message to match Desktop message

Added keys.svg icon for password dialog. License: CC-BY-SA 3.0. Copyright by Dan Chapman <dpniel@ubuntu.com> 2014.