Avoid doing expensive work when not mapped

Avoid doing expensive work when not mapped

For some unknown reason we were connecting to app-added and
app-removed on ShellAppMonitor in the AppDisplay class, which
never made any use of the data.  Simply don't connect to those
signals for now.  In the future we should have AppDisplay
be using the AppIcon class which will more correctly handle
dynamic changes.

In the AppWell, avoid doing the full relayout until we're
actually mapped.

https://bugzilla.gnome.org/show_bug.cgi?id=597169