#2708 Timed animation: delay between objects in a group

timed animation with enable-personality=true causes a small but visible delay between objects belonging to a group. This delay, like the normal branch-duration, scales with the time speedup/slowdown.

Example: run these animations (at 1/16x time speed to see the delay better).

 <animation>
  <name>GroupA</name>
  <object-name>ObjA1</object-name>
  <object-name>ObjA2</object-name>
 </animation>

  <animation>
  <name>GroupB</name>
  <object-name>ObjB1</object-name>
  <object-name>ObjB2</object-name>
 </animation>

  <animation>
  <type>timed</type>
  <object-name>GroupA</object-name>
  <object-name>GroupB</object-name>
  <use-personality type="bool">true</use-personality>
  <branch-duration-sec>0.1</branch-duration-sec>
  <branch-duration-sec>0.1</branch-duration-sec>
 </animation>

Expected: All objects in GroupA as well as in GroupB appear/disappear simultaneously.
Result: the switching between ObjA2 and ObjB2 happens slightly later than ObjA1 and ObjB1.