GUI: fix sendcoinsdialog/sendcoinsentry

Rewrite of sendcoinsentry.ui for payment protocol introduced a little design error on KDE 4, Qt 4 here.
Adding a stretch in sendcoinsdialog fixes the problem for me.

Screenshot: http://img51.imageshack.us/img51/4319/db2l.png

I also tried layoutRowStretch in sendcoinsentry directly, but this does not work. Did not look at payment protocol.