appDisplay: Show label when frequent app view is empty.

Currently we show a empty view, that seems broken, so we add a label
showing that there's not enough frequent applications to show.

https://bugzilla.gnome.org/show_bug.cgi?id=694710
appDisplay: Default to All view when not enough usage data is available

The frequent view is not useful when it doesn't contain any applications
yet. While the previously added label makes this state appear less like
an error (OMG, my apps are gone!), it doesn't address the issue of
usefulness - default to the more helpful All view in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=694710