GUI: Ctrl+Q shall exit, not just close the window

Thanks to Brian Jackson <iggy@theiggy.com> (iggy on IRC) for a bugreport.
GUI: validate each address separately and also when filled in programatically

Previously, the validation was performed based on the contents of all items.
This meant that after a single field contained invalid data, any subsequent
modification to any other field would render the newly modified field in red.
That's wrong, the validation shall be based exclusively on data from the
modified field.

This patch also triggers the validator when setting the data programatically.

CCMAIL: sreepriya1111@gmail.com
GUI: Ignore the human-readable part when it's just a repetition of the actual e-mail address

This is more or less just a limitation of the current address parsing code --
the dialog denies addresses like "foo@example.org <foo@example.org>".