demo: Make GatSidebar demo more interesting
demo: Make GatSidebar demo more interesting

This adds some simple fixes, such as sizing, and allowing vertical
scrolling on our sidebar widget. More items are used by default and
the titles have been made a little clearer, providing the foundation
for a slightly-less-boring-than-paint-drying-on-a-sunday demo.

ikeydoherty/gnome-app-templates,ikeydoherty/gnome-app-templates