Changed speed dropdowns in CustomAnimationPane and CustomAnimationDialog to Duration combobox.
Used Custom Widget VclComboBoxNumeric with MetricBox class.
Unit set in seconds.

Change-Id: I86e18da77dcdd72d429a2c02d56aa4b1cf5f6c54
Reviewed-on: https://gerrit.libreoffice.org/23157Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>