fdo#35712 fdo#40315 fdo#40314 Autoselect trendline type

Modification of Degree or Period select trendline type
Modification of Intercept select option

Change-Id: If3a212c381061e831ee4411c8e17fbaf8ca12895
Reviewed-on: https://gerrit.libreoffice.org/6815
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>