Add argument for the mail address again.

This argument had been removed in 3ce2855ab7abe1e7638c1d80dfb9cbf1ffcd6d59
Since then it has not been possible to use ksendemail to open
the messagecomposer with a prefilled "To: " field apart
from suppling the full Header.

This enables ksendemail again to function as the "default mail
application" on Windows correctly and open a Messagecomposer window
with a prefilled To: field (e.g. when opening mailto urls)