qt, refactor: Convert SendCoinsEntry to a sub-QWidget

It is no longer needed to derive the `SendCoinsEntry` class from the
`QStackedWidget` one.

src/qt/forms/sendcoinsentry.ui

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <ui version="4.0">
  <class>SendCoinsEntry</class>
- <widget class="QStackedWidget" name="SendCoinsEntry">
+ <widget class="QWidget" name="SendCoinsEntry">
   <property name="geometry">
    <rect>
     <x>0</x>
@@ -16,10 +16,6 @@
   <property name="autoFillBackground">
    <bool>false</bool>
   </property>
-  <widget class="QFrame" name="SendCoins">
-   <property name="frameShape">
-    <enum>QFrame::NoFrame</enum>
-   </property>
    <layout class="QGridLayout" name="gridLayout">
     <property name="topMargin">
      <number>8</number>
@@ -218,7 +214,6 @@
      </widget>
     </item>
    </layout>
-  </widget>
  </widget>
  <customwidgets>
   <customwidget>

src/qt/sendcoinsentry.cpp

@@ -20,7 +20,7 @@
 #include <QClipboard>
 
 SendCoinsEntry::SendCoinsEntry(const PlatformStyle *_platformStyle, QWidget *parent) :
-    QStackedWidget(parent),
+    QWidget(parent),
     ui(new Ui::SendCoinsEntry),
     model(nullptr),
     platformStyle(_platformStyle)
@@ -31,8 +31,6 @@ SendCoinsEntry::SendCoinsEntry(const PlatformStyle *_platformStyle, QWidget *par
     ui->pasteButton->setIcon(platformStyle->SingleColorIcon(":/icons/editpaste"));
     ui->deleteButton->setIcon(platformStyle->SingleColorIcon(":/icons/remove"));
 
-    setCurrentWidget(ui->SendCoins);
-
     if (platformStyle->getUseExtraSpacing())
         ui->payToLayout->setSpacing(4);
 
@@ -227,7 +225,7 @@ void SendCoinsEntry::changeEvent(QEvent* e)
         ui->deleteButton->setIcon(platformStyle->SingleColorIcon(QStringLiteral(":/icons/remove")));
     }
 
-    QStackedWidget::changeEvent(e);
+    QWidget::changeEvent(e);
 }
 
 bool SendCoinsEntry::updateLabel(const QString &address)

src/qt/sendcoinsentry.h

@@ -7,7 +7,7 @@
 
 #include <qt/sendcoinsrecipient.h>
 
-#include <QStackedWidget>
+#include <QWidget>
 
 class WalletModel;
 class PlatformStyle;
@@ -22,10 +22,8 @@ namespace Ui {
 
 /**
  * A single entry in the dialog for sending bitcoins.
- * Stacked widget, with different UIs for payment requests
- * with a strong payee identity.
  */
-class SendCoinsEntry : public QStackedWidget
+class SendCoinsEntry : public QWidget
 {
     Q_OBJECT