Bug 836647 - Set newly created tabs to background by default. r=benfrancis

Newly created mozbrowsers default to foreground, if we open tabs
using the context menu we never set them to background leaving
a lot of fg processes. This sets them to background by default, we
already ensure to set them to foreground every time they are
needed to be so.
Merge pull request #8198 from daleharvey/default-background-tabs

Bug 836647 - Set newly created tabs to background by default