[REVIEW: 4-0 3-6] impress doesn't copy custom styles

Hi all,

when a slide that uses custom styles is copied to another presentation,
the styles are not copied, possibly breaking the slide's layout.
Commit 24578b804007d8c3201e5ed32b8485e1725c33c1 fixes that. I would like
to get it into 3.6 and 4.0 (AFAICS it has never worked, so it is not a
regression).

How to test:
1. create a presentation
2. open styles & formatting window
3. switch to graphic styles
4. create a new style (change text color or something similar easily
   visible)
5. insert a text box and write something
6. apply the style to the text
7. copy the slide to clipboard
8. create another presentation
9. paste the slide into it

D.