Increase default window size

The current default window size of 750x500 is quite small on today's
screens, and results in an initial user experience of a very cramped app
window. The very first action that I see from new users interacting with
Dolphin is to make the window bigger.

So let's do it for them by making the default window size a bit bigger
ourselves.

src/dolphinmainwindow.cpp

@@ -178,7 +178,7 @@ DolphinMainWindow::DolphinMainWindow() :
     if (firstRun) {
         menuBar()->setVisible(false);
         // Assure a proper default size if Dolphin runs the first time
-        resize(750, 500);
+        resize(760, 550);
     }
 
     const bool showMenu = !menuBar()->isHidden();