Flowchart updates - added/fixed some display nodes
Merge from Kratz:
- documentation updates
- ComboBox class (updateList() allows entire item list to change while remembering previous setting)
- HistogramLUTItem avoids using lookup table if possible
- GradientEditorItem has a method to ask whether the gradient is trivial (can be applied without the use of a lookup table)
- ROI, LinearRegionItem, InfiniteLine no longer redraw for every mouse movement
- Corrected ImageItem.setRect transformation order
- PlotCurveItem uses nkPen for interpreting shadowPen arguments
- PlotItem and PlotWidget wrap a few more missing methods from ViewBox
