libdeng2|Clock: Priority audience for time change

In some cases it is necessary for certain time change audience members
to be notified before other members (e.g., ScalarRule must be already
invalidated when the regular time change notifications go out).

Added a "priority" audience to Clock that will always be notified
before the regular audience. Animation-related clock observers are now
put in the priority audience.
UI|Console|LogWidget: Fixed animation glitches

Drawing must occur using the same values that were used when the
geometry was updated.