Bitcoin-Qt: translation fixes in sendcoins

- remove some unneded translatable strings from sendcoinsentry.ui file and
  rename some elements for better readability
- optimize string prorcessing in SendCoinsDialog::on_sendButton_clicked()
- make all UI labels for secure payments plain text and move the settings
  to sendcoinsentry.ui file
- remove some unneeded button and default button definiton from warning
  message boxes