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
Merge pull request #2991 from Diapolo/translation_fixes

Bitcoin-Qt: translation fixes in sendcoins