GUI: Detach the composer from mainwindow

This prevents me from doing stupid things (like writing a giant mail, forget
about it on another VD and close the mainwindow ...)
GUI: Flash red on search failure

This tells me there's been in error when typing something stupid like eg.
":=subjet 12345"

Ported-by: Jan Kundrát <jkt@flaska.net>
GUI: Don't switch between the multipart/alternative parts by mouse wheel

This prevents me from accidentally seeing ugly html mails ;-)

Ported-by: Jan Kundrát <jkt@flaska.net>
Thomas contributed this year as well
Make sure that e-mail recognition code can accept any atext in the local-part of the e-mail address

It's rather embarrassing when the code cannot parse mlmmj-generated Reply-To :(.
Experiment with the Travis Continuous Integration