GUI: place focus on the first sender when composing a new mail and to the body when replying

Thanks to Sebastian Spaeth <Sebastian@sspaeth.de> for reporting.

fixes #635
GUI: remove ambiguous Ctrl+N shortcut

This was introduced by me in commit:5ec815a3e15ba5939326f77f30024671232fb4eb.

Thanks to Sebastian Spaeth <Sebastian@sspaeth.de> for reporting.

fixes #631