The type of the widgets is already a Button so there's no need in
repeating that in the struct field.
Also remove some type annotations since the compiler can infer them
from the type of the struct fields.
I strongly believe that these do not belong there and should be moved
elsewhere. But for the time being and prototyping they are "good enough".
People most of the time tend to import from an OPML file only on the first
use of the App. Then the functionality sits there and is never used again.
That's why I think import should be moved into a preferences dialogs and
have the empty view suggest the action.
Exporting OPML files is also not that common, I also believe this should be
moved into a preference dialog instead of the hamburger menu.
Added a stack to switch between a normal and a BackButton view.
Also added a centered stack switcher and removed the Home button.
Backbutton is not wired yet.
