Renaming view to trailing and leading and adding rtl support.

[aloisdeniel]

I renamed view1 and view2 to trailing and leading, which I find more clear (we already have those terms in AppBar, GridTileBar, ListTile for example).

This PR also adds rtl support which means that leading and trailing views are inverted when the text direction of the application is rtl and the mode is horizontal.

[toshiaki-h]

Thank you for your PR. But I'm not going to accept this PR.
Because now I'm working on replacing view1 and view2 with children, but I don't think the leading and trailing fit into that name.
However, rtl thinks I need it too, so I'm thinking of take in that part.

[aloisdeniel]

Oh yes, a List<Widget> children is even better!

And Flex / Column / Row have a textDirection property which makes rtl easy to implement.