fdo#40316 Remove SpinButton in UI for extrapolate trendline

UI Trendline: Spinbutton are not adapted for real value of x-values used
in extrapolate For/Back-ward. Replaced with FormattedFiled
like Intercept value

Change-Id: Idc49f842a0f76906eb3c027e15a493e625c3d535
Reviewed-on: https://gerrit.libreoffice.org/7067
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>