Move hook_fences_suggestion_info implementations to an .inc file

Our list of theme hook suggestions is getting quite long. When I started to implement the "region" suggestions, the fences_fences_suggestion_info() implementation was just shy of 400 lines long by itself. Half the .module files's length!

We can add a hook_hook_info() implementation that defines a "fences" group so we can put our fences_fences_suggestion_info() in a fences.fences.inc file and it will automatically be included when that hook is invoked.