Inspector: Support extending the inspector using GIOModules

This allows external modules to add a page to the Gtk Inspector.

https://bugzilla.gnome.org/show_bug.cgi?id=740983
Fix some oversights with header guards

Pointed out in https://bugzilla.gnome.org/show_bug.cgi?id=741254
Fix another header guard oversight

Pointed out in https://bugzilla.gnome.org/show_bug.cgi?id=741252