preview-nav-buttons: Always evaluate their visibility when showing

Otherwise the previous and next buttons are not hidden when you reach
the beginning or end while navigating within the preview.
preview-nav-buttons: Remove redundant check

PhotosEmbed is already careful not to call
photos_preview_nav_buttons_set_model when we are navigating within the
preview. Therefore this check is not that useful.
dlna: Update DBus signature after dleyna-renderer ABI fix

dleyna-renderer has fixed some of its method and signal signatures to
correctly mark object paths with the 'o' type instead of plain strings
(see https://github.com/01org/dleyna-renderer/commit/3a225c3).

Even if no proper code change would be needed as both 's' and 'o' are
represented as gchar*, this unfortunately means that the generated DBus
methods will fail to validate the incoming data.

Since this is a new feature not yet part of any stable release and since
Photos gracefully handles the validation error by disabling the feature
on startup, no proper transition has been put in place and having a
small flag day seemed an acceptable choice.

https://bugzilla.gnome.org/show_bug.cgi?id=706502