Enable HiDPI support

Reference: issue #3
(cherry picked from commit 63d2dfdf9c)

src/danbooru_client.cpp

@@ -67,5 +67,8 @@ int main(int argc, char *argv[])
     window->setObjectName("danbooruMainWindow");
     window->show();
 
+    // Enable HighDPI support
+    app.setAttribute(Qt::AA_UseHighDpiPixmaps, true);
+
     return app.exec();
 }