Don't show welcome dialog if using shell to open .gfxtrace file

At least on macOS, if you double click a `.gfxtrace` file in finder, you'll have GAPID launch (typically without focus?) and show you the welcome screen (Open File, Open Recent, Capture) while the trace loads in the background.

We should make sure the client comes to focus (it'll often silently open in a different macOS 'desktop' for me) and hide the welcome screen.