Avoid code duplication for the filling properties of worksheet elements
Right now we're copy&pasting a lot of code for backround and filling properties across the multiple classes. Similar to how the code was simplified and unified for symbols (Symbol and SymbolWidget classes), we should also do it for the background properties.
Relevant for:
- worksheet background
- plot area background
- plot legend
- xy-curve filling
- histogram filling
- box plot filling
Once we have this re-usable classed for the background properties, it should be also easier to implement the different coloring for the different datasets within a bar or box plot.
Closes #240 (closed)
Edited by Alexander Semke