🍒applets/taskmanager: reset animating property on mouse enter

TaskList Grid move during a SequentialAnimation sets an later removes
"animating" property. Sometimes (e.g. session restore) this animation
fails to complete and then "animating" property remains true. This
prevents drag'n'drop with mouse, as MouseHandler onPositionChanged
skips everything if "animating" property is set.

BUG:465377
FIXED-IN: 6.0

(cherry picked from commit 67886ca2)