ssh-askpass: Set window type to dialog

On i3 window manager, ssh-askpass window is created as
full/top level window instead of floating window by default.
i3 sets dialog, utility, toolbar and splash windows to floating.

Changes ssh-askpass window type to dialog

Reported-By: aszkid via github.com in #207

Signed-off-by: Minarto Margoliono <lie.r.min.g@gmail.com>