GsShellDetails: Add a getter for the app
GsShell: Improve back stack

Future-proof this a bit, by also supporting going back
to an app page - this may be used in the future when we
get 'related' links on the app details page.
Improve handling of category

Keep track of the current subcategory on the category page,
and go back to that subcategory when coming back from an app
page.
Improve scrolling

Arrange things so that we scroll to the top only when switching
pages using the main buttons, not when going back from a category
or app page.