Fix crash opening folder (Debian bug #771737)

See https://bugs.debian.org/771737 for details.
Compose window: activating the GtkEntry with sender address activates the account selection combobox. Closes bug #3345, reported by  Barry Warsaw.
Properly allocate buffer in conv_encode_header_full() for quoted-printable encoding. Closes bug #3338, reported by Michael Hughes.
Compose window: sender address line gets its own label with a mnemonic, similar to Subject line.
Compose window: Change Subject mnemonic from 's' to 'u', since 's' is already used by Spelling menu.
when cancelling a msg written with an external editor, get rid of the draft copy. thanks to Darko Koruga.
fix bug 3351, 'Two problems when re-sending a file with an attachment' (really one problem, though)