Support TabBarView swipe
Support TabBarView swipe

Swiping left or right in a TabBarView now changes the selected tab in the way it's supposed to.

Currently swipe gestures that start while the selection change is underway are ignored. Will fix that in a separate change.

Hixie/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,aghassemi/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,aghassemi/flutter,cbracken/flutter,turnidge/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,aghassemi/flutter,aghassemi/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,turnidge/flutter