[Qt] add defaultConfirmTarget constant to sendcoinsdialog

- replaces some hard-coded values for the default confirmation target
- also simplify code that is using the new constant

src/qt/sendcoinsdialog.h

@@ -23,6 +23,8 @@ QT_BEGIN_NAMESPACE
 class QUrl;
 QT_END_NAMESPACE
 
+const int defaultConfirmTarget = 25;
+
 /** Dialog for sending bitcoins */
 class SendCoinsDialog : public QDialog
 {