Bitcoin-Qt: translation fixes in sendcoins

• remove some unneeded 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 unneeded button and default button definiton from warning message boxes
• remove fixed font-size when sending coins to an address with label and use monospace font for addresses