New routine dt_gui_store_last_preset to share code.
Record last preset name when creating a new one.

This fixes a usablility issue where it was proposed to update the
preset selected before the newly created one. It was then possible
to erase an old preset.
Merge branch 'add-update-presets'

* add-update-presets:
  Record last preset name when creating a new one.
  New routine dt_gui_store_last_preset to share code.