Custom Animation Dialog: Enabled dynamic update of time in ComboBox

Change-Id: I42e83486f6eea2684fb9a4f45981bd2f3d3336ab
Reviewed-on: https://gerrit.libreoffice.org/24465Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>