gui: Move static placeholder texts to forms

There was an issue around the time of Qt 4.6 when placeholder text was
introduced, that caused a compile failure when it was specified in the
form.

As a workaround the placeholder texts were moved to the code.

Qt 4 hasn't been relevant to us for ages. So move all (non-parametrized)
placeholder texts to the form files instead.

It's better to keep this kind of text content together. Makes sure
translate/no-translate status is kept as it is.

src/qt/optionsdialog.cpp

@@ -108,8 +108,6 @@ OptionsDialog::OptionsDialog(QWidget *parent, bool enableWallet) :
             ui->lang->addItem(locale.nativeLanguageName() + QString(" (") + langStr + QString(")"), QVariant(langStr));
         }
     }
-    ui->thirdPartyTxUrls->setPlaceholderText("https://example.com/tx/%s");
-
     ui->unit->setModel(new BitcoinUnits(this));
 
     /* Widget-to-option mapper */