[components] Add Nemo's ApplicationWindow and rotation handling

We may consider renaming the component, but anyway:
ApplicationWindow includes a page stack (StackView) which handles
the pages of the application.

ApplicationWindow includes an Item handling UI rotations,
and offers allowedOrientations (and other useful properties)
to help the dev handle rotation locking.
[examples] Update components gallery
Merge pull request #8 from faenil/applicationwindow

Add ApplicationWindow, ui rotation handling, and update examples accordingly