Enable high-DPI scaling to support 4k screens.

This enables coordinate system scaling for high-DPI
displays, which enforces correct proportions even
on small 4k displays. The icons are scaled up without
interpolation, which makes them crisp, but a bit
pixelated. A new scalable icon set will solve this
problem, but is not scope of this patch.

Resolves #548, #1381, #1710, #1888

In addition, this patch enforces the KeePassXC icon
theme for the KMessageBox close icon, since using
the system theme produces very ugly icons on some
Linux systems.

src/gui/KMessageWidget.cpp

@@ -94,7 +94,7 @@ void KMessageWidgetPrivate::init(KMessageWidget *q_ptr)
     QAction *closeAction = new QAction(q);
     closeAction->setText(KMessageWidget::tr("&Close"));
     closeAction->setToolTip(KMessageWidget::tr("Close message"));
-    closeAction->setIcon(FilePath::instance()->icon("actions", "message-close"));
+    closeAction->setIcon(FilePath::instance()->icon("actions", "message-close", false));
 
     QObject::connect(closeAction, SIGNAL(triggered(bool)), q, SLOT(animatedHide()));
 

src/main.cpp

@@ -46,6 +46,10 @@ Q_IMPORT_PLUGIN(QXcbIntegrationPlugin)
 
 int main(int argc, char** argv)
 {
+#if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
+    QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+#endif
+
     Application app(argc, argv);
     Application::setApplicationName("keepassxc");
     Application::setApplicationVersion(KEEPASSXC_VERSION);