ContinuousPaletteGenerator now returns a (fixed) color for nan's
ScatterPlotGraph: Optimized compute_color, assigns colors to missing values
ScatterPlotGraph: Added a shape for missing data
ScatterPlotGraph: Handling missing data in sizes
ScatterPlotGraph: code cleanup
ScatterPlotGraph: minor fixes
Fix failing setting tests.

- Manually initialize subprovider settings, as mock widgets/components do
  not do that.
- Open context before trying to move settings from/to widget with
  context handler.
Test settings when setup.py test is run.
Removed Varible.var_type from base Orange.
Removed var_type from tests.
Removed use of var_type from widgets.
Tricked test_settings to work without VarTypes.
Fixed pull request.