[sailfish-browser] Virtualize background tabs when another application is opened

Current implementation virtualizes backgrounded tabs when low memory
notification (warning / critical) is received even if the browser is
at foreground. Virtualizing backgrounded tabs when another application
is brought foreground guarantees that there are no background tabs
when another application is running regardless of low memory
notification.
Oops. This sentence got copied here as well. Should not be here.

One other way to implement this kind of early release would be to do this when screen is dimmed meaning that browser goes to background and itself is not requesting screen to be on.