Bug 1077652 - Hide xul:panels by default when not opened r=gijs

We need to hide <xul:panel>s by default for performance reasons. The RefreshDriver calls gtk/nsWindow::GetClientOffset() in a hot loop and causes a lot of communication between us and the XServer. We and up poll()ing and waiting for XServer responses in RefreshDriver::Tick() which regresses performance on Linux by 50-100%.
Bug 1077652 - Link a preloaded <xul:browser> to a newly created tab instead of swapping docShells
Bug 1077652 - Remove old BrowserNewTabPreloader and fix tests