statistics: collect colors in a StatsTheme class

To enable rudimentary theming, collect all colors in a new
theme class. The class has to be passed down to the various
items.

In general the items save a reference to the them in the
constructor. Alternatively, they might also just query
the StatsView everytime they need to access a color.
For now, it's hard the say what is preferred: a reference
per item or a function call per invokation?

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

stats/statsseries.cpp

@@ -1,9 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "statsseries.h"
 #include "statsaxis.h"
+#include "statsview.h"
 
 StatsSeries::StatsSeries(StatsView &view, StatsAxis *xAxis, StatsAxis *yAxis) :
-	view(view), xAxis(xAxis), yAxis(yAxis)
+	view(view), theme(view.getCurrentTheme()), xAxis(xAxis), yAxis(yAxis)
 {
 }