* Allow apps to change the title margin in app bar.

* Fix documentation links and the shouldLayout function in the _ToolbarLayout

* Rename margin to spacing

* review comments

* Test fixes. Now we also test the width to make sure enough space is left for trailing widget

* Expose the middle spacing default and use it in app bar.

* Fix analyzer break

* Doc fixes due to review

* Selectively apply margin between middle and leading

If ToolbarLayout does not have a leading widget, we should not put a margin between leading and middle areas since it ends up being blank space that looks odd. Fixes https://github.com/flutter/flutter/issues/11963

* Fix the failing test which is a good test case for missing leading widget

Fixes #11381

* works using pageview
it’s a bit heavy and scrolls
no documentation or test yet

* docs and tests

* more docs tweak

* revert drive by page view changes

* swap out the indexed stack for a custom thing

* layout everything on the cheap
tests pass

* add more tests

* move back to stack and add docs

* review notes

* build all the tabs once built

Extract layout logic in material app bar to a common file that can be reused for cupertino