Merge bitcoin#17195: gui: send amount placeholder value

57e2ede Send amount shows minimum amount placeholder (JeremyCrookshank)

Pull request description:

  Noticed that there wasn't a default value for the send amount. However if you put a value in or click the up and down arrows you're unable to get it blank again, so it makes sense that it has a default value. I hope this also makes it more clear that users can send less than 1 BTC if it shows the 8 decimal places

  PR:
  ![Capture](https://user-images.githubusercontent.com/46864828/67132088-549c6180-f1ff-11e9-9ba5-67fdcd6db894.PNG)

ACKs for top commit:
  promag:
    ACK 57e2ede.
  GChuf:
    ACK 57e2ede
  laanwj:
    ACK 57e2ede, this is a surprisingly compact solution too

Tree-SHA512: 354590d2a88231b8649f7ae985c8a7864d74ca0e1f8603cb1730ba46747084de90ee6285ce4d39ee04b054fb9cd2d78ebc71146f3af694c37a8a3aff7f051800

Author: laanwj

src/qt/bitcoinamountfield.cpp

@@ -102,7 +102,7 @@ class AmountSpinBox: public QAbstractSpinBox
         CAmount val = value(&valid);
 
         currentUnit = unit;
-
+        lineEdit()->setPlaceholderText(BitcoinUnits::format(currentUnit, m_min_amount, false, BitcoinUnits::separatorAlways));
         if(valid)
             setValue(val);
         else