show a blank page while waiting for a new page after switching back to *w3m* buffer

[=?utf-8?b?56mN5Li55bC8?= Dan Jacobson <jidanni@xxxxxxxxxxx>]

I note that I browse something (X), then switch to reading my mail or
something, then do e.g., (w3m-browse-url (Y)) first emacs-w3m switches
to the *w3m* buffer, and then it fetches the webpage and displays it.

That works fine except on slow computers, where the user will end up
looking at X again for several seconds before finally being shown Y.

He thinks, huh? is this Y? No... oh it is X.

Therefore maybe a blank page should be shown instead of X.