When clicking or using keyboard to activate the menu item System::LoadCartridge, invoke the file browser's FileOpen functionality in cartridge browsing mode. If a file were chosen, pass the selected file name onto the Cartridge loading class to begin playing the game.
I provide a callback function inline from within another inline callback function. You can literally pass anonymous functions around as function arguments inside of anonymous functions. If that's not the coolest thing you've ever seen, then you suck :P