weston_surface::committed is not as it seems

The following discussion from !627 (merged) should be addressed:

• @emersion started a discussion: (+2 comments)

  Note to self: wsurface->committed is populated when assigning a role. It's a callback, not a bool.

  (and @zzag):

  I suspect that other compositor developers may think the same way when they see this code ("committed" is a boolean, not a callback). Maybe leave a comment mentioning the fact that committed is not what it seems?

I mean we should just rename it to something more descriptive.