Bitcoin-Qt: translation fixes in sendcoins

- make life for translators easier and try to remove %1 and such in
  sendcoinsentry and remove some unneded translatable strings from .ui
  file
- optimize string prorcessing in SendCoinsDialog::on_sendButton_clicked()
- remove some unneeded button and default button definiton from warnings
- rename some sendcoinsentry.ui file elements for better readability