Allow Enter key to select recent DB on OS X

* Override `keyPressEvent`on WelcomeWidget on OS X to `openDatabaseFromFile`.

openDatabaseFromFile is already invoked via the QListWidget::itemActivated signal,
  but this signal doesn't fire on OS X for Enter.
QListWidget::itemActivated activates on an OS specific activation key. [1]
On Windows/X11, this is Enter, which lets the user easily
  navigate with just the keyboard.
On OS X, this is Ctrl+O, which is already bound to Open Database. This means that itemActivated cannot fire via the keyboard.

Per StackOverflow [2], the recommended solution is to catch
the enter/return key press manually.

This seems like a common problem with Qt. [3] [4]

[1] https://doc.qt.io/archives/qt-4.8/qlistwidget.html#itemActivated
[2] https://stackoverflow.com/questions/31650780/when-does-a-qtreeview-emit-the-activated-signal-on-mac
[3] https://forum.qt.io/topic/36147/pyside-itemactivated-not-triggered-on-mac-os-x-with-return-key
[4] https://github.com/dolphin-emu/dolphin/pull/6099

src/gui/WelcomeWidget.cpp

@@ -16,6 +16,7 @@
  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <QKeyEvent>
 #include "WelcomeWidget.h"
 #include "ui_WelcomeWidget.h"
 
@@ -76,3 +77,11 @@ void WelcomeWidget::refreshLastDatabases()
         m_ui->recentListWidget->addItem(itm);
     }
 }
+
+void WelcomeWidget::keyPressEvent(QKeyEvent *event) {
+    if (m_ui->recentListWidget->hasFocus() && (event->key() == Qt::Key_Return || event->key() == Qt::Key_Enter)) {
+        openDatabaseFromFile(m_ui->recentListWidget->currentItem());
+    }
+
+    QWidget::keyPressEvent(event);
+}

src/gui/WelcomeWidget.h

@@ -43,6 +43,9 @@ signals:
     void importKeePass1Database();
     void importCsv();
 
+protected:
+    void keyPressEvent(QKeyEvent *event) override;
+
 private slots:
     void openDatabaseFromFile(QListWidgetItem* item);