When we launch Ladybird, we currently:
1. Create a BrowserWindow, whose constructor navigates to the configured
new tab page URL.
2. Then navigate to either "about:blank" or whatever URL is provided via
the command line.
This patch removes step 2 and forwards the URL from the command line (if
any) to BrowserWindow. BrowserWindow's constructor then either navigates
to that URL or the new tab page URL.