rework the update machinery to use callbacks

rework the update machinery to use callbacks

Besides improving performance when updating stuff, we ideally have no
text object specific code in update_stuff() anymore (aside some
leftovers).

The macros in construct_text_object() have gotten a bit crazier than
they were before:
* using CALLBACK(&func) instead of an INFO_* parameter to OBJ() will
  make it add the given callback to the list of callbacks to be iterated
  over at each update interval.
* BEWARE: the above assumes function pointer values to be > 0!
* This implicitly fixes a bug in the code: passing 0 as INFO_* value
  led to selecting INFO_MAIL (1 << 0 == 1).
* Now it would select INFO_CPU (== 0), which got unused and therefore is
  not a problem at all (the 0 value should be unused in enums anyway).

This needs some more work, then we should be able to drop the whole
INFO_* enum. Then CALLBACK() can die again and with it goes the ugly
casting stuff done to distinguish callbacks from INFO_* values.