DRY-cleanup RegressionCurveItemConverter

Change-Id: I8fb799b2efc81587252161ccadfa1dbd6b096bda
Simplify conversion from/to ItemSet/PropertySet for RegressionCurves

Change-Id: I5ddf53f984508a88fd02063eb9f81cd05cf627c9
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
Support regression curve name in ODF.

Change-Id: Ic065d63eee507906febfa15eb6d5ae1e7de58280
Support regression curve name in OOXML (import & export).

Change-Id: I847e0df4f160e4b5078961a0e77c1e1e3fff9bd4
Fix detection of force intercept in XLS 97/2000/XP format.

NaN value indicates if the force intercept is enabled or not.

Change-Id: I51d33a6e04a4b5832f241e7ae7ee6d2fbb32ea9b
Support regression curve name in old binary XLS XP/2k/2k3 format.

Change-Id: Ie6f2fd3cce70e4789805e788846d3beb64620a4b