appDisplay: Add additional view showing frequently used apps

According to the design mockups, the app picker should follow the
recent view pattern as used by applications, where the user is first
offered a subset of applications he/she is likely to start, and only
then allow switching to the full set of installed applications.
So implement the ability to manage several views in AppDisplay and add
FrequentView as additional view, which uses the existing ShellAppUsage
to display a list of frequently used applications.

https://bugzilla.gnome.org/show_bug.cgi?id=694192