Add "extra" parameter for the widget definition states with the
default set to "any". For tab item, the extra parameter can be
"first", "last", "middle" and "first_last" (only one). This is
needed to draw first and last tab item differently.

Change-Id: I46b6897b485b4df94ab6fe9521925c3715eaa24e
Reviewed-on: https://gerrit.libreoffice.org/68817
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>