Problem can be seen with non-gtk3, if the initial
palette has no scrollbar (e.g. "standard" palette).

The cause is that in non-gtk3 we don't have overlay
scrollbars, so the palette area needs to be enlarged
when switching from a palette with a scrollbar to a
palette without a scrollbar. In practise, this was
happening also on initial show, although the palette
already had the correct width.

To fix that, start with a never scrollbar policy by
default, and add the scrollbar later if needed.

Change-Id: I5286f301b3c7ef5c72b650290ace784222f7922d
Reviewed-on: https://gerrit.libreoffice.org/56995Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>
Tested-by: Maxim Monastirsky <momonasmon@gmail.com>