When Impress starts, the dialog box propose to either create an empty
presentation, create one from a template or open a existing one. In this later
case, the last button of the dialog still display "create". One may think it
will create a new presentation based on an existing one instead of just opening
the selected existing presentation.

The button must be rename to something like "Open" when "Open existing
presentation" is selected and rename back to "Create" if the user select either
"Empty presentation" or "From template".