fdo#40314 Free value of intercept for trend line

Intercept value field has been changed to FormattedField
to support scientific notation and more than 4 digits
after decimal point

Change-Id: Idb2d27862ddabcdc96a036d93f1f3498cc6327df
Reviewed-on: https://gerrit.libreoffice.org/6904
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>