sidebar: Track title changes and own meta for widgets
sidebar: Track title changes and own meta for widgets

We now track our own items and the GtkStack pages to ensure we sync
title, etc. The sidebar items now display the title as dynamically
set via the "title" child property, and we store internal information
in a GHashTable for easy access.

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