fdo#40315 Improve UI for moving average trendline

If moving average trendline is selected, show equation is
uncheked, and all options disable

Change-Id: Ic25c09b8c8410a12ac9f14cb0ba7833f304053ee
Reviewed-on: https://gerrit.libreoffice.org/6796
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>