fixes the 'previousTab' logic, and uses a centralized 'DefaultTab' value
fixes the 'previousTab' logic, and uses a centralized 'DefaultTab' value

Instead of setting a magic '.Stream' in multiple places,
I added a '.DefaultTab' static property to the ElloTab enum.
Also, the 'previousTab' value was being set incorrectly (in didSet
instead of willSet)

ello/ello-ios,ello/ello-ios,ello/ello-ios,ello/ello-ios