Add macros/functions to help name timeouts.

The macros form a name for the timeout GSource implicitly.

New macros:

  e_named_timeout_add()
  e_named_timeout_add_full()
  e_named_timeout_add_seconds()
  e_named_timeout_add_seconds_full()

New functions (called by the macros):

  e_timeout_add_with_name()
  e_timeout_add_seconds_with_name()