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
- Added rate-limited mode to SignalProxy
- Added basic text justification to LabelItem
- ViewBox.addItem now has ignoreBounds option, which causes the item to be ignored when autoscaling
- Added ValueLabel widget
- Fixed some autoscaling bugs
- InfiniteLine fix - no hilight if movable=False
