Copy changes from ACQ4:

ExceptionHandler: avoid generating new errors on top of incoming recurion errors
Canvas.removeClicked: remove all selected items
CSVExporter: minor fixes
SVGExporter: support for exporting gradients
Flowchart: fix for unicode filename support
    added Filters.RemoveBaseline Node
GradientEditor: Add units to axis values, context menu for setting precise tick values
PlotDataItem: speed fixes for loading MetaArray (need to test)
    make automatic downsampling factor user-adjustable
Fixed ScaleBar (needs testing)
ViewBox: check for invalid range in updateAutoRange
ComboBox: better behavior for wide item names on OSX
copied crosshair ROI changes from ACQ4