Try to make SSHAskPass topmost dialog

(fixes issue #2020)

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Signed-off-by: Sven Strickroth <email@cs-ware.de>
Correctly restore maximized but minimized window

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>