Simplify conversion from/to ItemSet/PropertySet for RegressionCurves

Change-Id: I5ddf53f984508a88fd02063eb9f81cd05cf627c9
Reviewed-on: https://gerrit.libreoffice.org/6872
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>
Custom name for a trend line (shown in legend).

Previously, a trendline name was fixed to a combination of the
trendline type and the name of the series. With this it is possible
to add a custom name for the trendline, which is shown in the
legend.

Change-Id: I61eaaf14e6df45088abdf1f3e9c24877d26ae8a7
Reviewed-on: https://gerrit.libreoffice.org/6873
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>