Disable up button when viewing root view

The up button in the toolbar only works when there's at least two views
stacked in the window. It doesn't do anything when there's only one
view, since there's no previous view that it can go back to. This can be
a bit confusing. Disable the button when it can't be used.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>