1058383: widgets are added and removed dynamically

This is better than using nested containers and using show/hide
because we allow the GtkTreeView to handle its own scrolling,
while the GtkLabel is handled by a GtkViewPort

With a container housing multiple widgets, one could move the
selected subscription row (with arrow keys) beyond the top or
bottom of the scrollable pane.
Merge pull request #918 from candlepin/ckozak/scrolling_allsubs

Ckozak/scrolling allsubs