Add visual accenting for the 'Create new receiving address' button

Summary:
> qt: Make "Create new receiving address" default unconditionally
>
>  This change makes this button visually accented, and gives to users a
> hint about the primary action.
> This commit does not change behavior.

> qt: Leverage the default "Create new receiving address" button
>
> When Enter or Return is pressed the default button will be always
> clicked. All buttons can always be clicked from the keyboard by pressing
> spacebar when the button has focus.

This is basically a simplification of the way pressing return in the
"Receive" tab of bitcoin-qt submits the form, doing it the standard Qt
way without additional code.

This is a backport of [[bitcoin-core/gui#39 | core-gui#39]]

Test Plan:
`ninja && src/qt/bitcoin-qt`

In the Receive tab of the main window, verify that pressing Return still
submits the form and creates a new address.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10135

Author: hebasto

src/qt/forms/receivecoinsdialog.ui

@@ -127,6 +127,12 @@
              <iconset resource="../bitcoin.qrc">
               <normaloff>:/icons/receiving_addresses</normaloff>:/icons/receiving_addresses</iconset>
             </property>
+            <property name="autoDefault">
+             <bool>false</bool>
+            </property>
+            <property name="default">
+             <bool>true</bool>
+            </property>
            </widget>
           </item>
           <item>

src/qt/receivecoinsdialog.cpp

@@ -248,20 +248,6 @@ void ReceiveCoinsDialog::resizeEvent(QResizeEvent *event) {
     columnResizingFixer->stretchColumnWidth(RecentRequestsTableModel::Message);
 }
 
-void ReceiveCoinsDialog::keyPressEvent(QKeyEvent *event) {
-    if (event->key() == Qt::Key_Return) {
-        // press return -> submit form
-        if (ui->reqLabel->hasFocus() || ui->reqAmount->hasFocus() ||
-            ui->reqMessage->hasFocus()) {
-            event->ignore();
-            on_receiveButton_clicked();
-            return;
-        }
-    }
-
-    this->QDialog::keyPressEvent(event);
-}
-
 QModelIndex ReceiveCoinsDialog::selectedRow() {
     if (!model || !model->getRecentRequestsTableModel() ||
         !ui->recentRequestsView->selectionModel()) {

src/qt/receivecoinsdialog.h

@@ -49,9 +49,6 @@ public Q_SLOTS:
     void reject() override;
     void accept() override;
 
-protected:
-    virtual void keyPressEvent(QKeyEvent *event) override;
-
 private:
     Ui::ReceiveCoinsDialog *ui;
     GUIUtil::TableViewLastColumnResizingFixer *columnResizingFixer;