message: [Qt] Don't set placeholder QPlainTextEdit
  `setPlaceholderText()` for QPlainTextEdit wasn't introduced until Qt 5.3.
  Guard with a simple pre-compiler conditional; needed to still support
  Qt 5.2.1 (Ubuntu Trusty system package version).
diff: @@ -61,7 +61,9 @@ SettingsSignMessageWidgets::SettingsSignMessageWidgets(PIVXGUI* _window, QWidget
  ui->labelSubtitleMessage->setText(tr("Message"));
  ui->labelSubtitleMessage->setProperty("cssClass", "text-title");
  +#if QT_VERSION >= 0x050300
  ui->messageIn_SM->setPlaceholderText(tr("Write a message"));
  +#endif
  ui->messageIn_SM->setProperty("cssClass","edit-primary");
  setShadow(ui->messageIn_SM);
  ui->messageIn_SM->setAttribute(Qt::WA_MacShowFocusRect, 0);
  