activatables: add 'specific_to' template parameter

The optional "specific_to" template parameter allows you to specify an
additional grouping.

You could have a, say, "framebuffer" class that is specific to a
graphics context (that instance can be set with the "set_specific_to"
protected member function). This way you can have multiple
framebuffers active at once in different graphics contexts.
graphics: add 'framebuffer' class