GUI: less scary placeholder text for the quicksearch widget

Thansk to Mark Constable <markc@renta.net> for his suggestion and to Thomas
Lübking for following up.

REVIEW: 110680
Refactoring: use better name

Right now the name is a bit misleading, but all confusion will be cleared by the
next commit.
Decouple attachment download from the lifetime of the enclosing GUI widget

Previously, the downlaod would get interrupted when the user switched to another
message -- and that would be rather counterintuitive.

REVIEW: 110681