Add SettingProviders.

SettingProvider is an abstraction taking care of reading and writing
settings of a class. They allow nesting of providers and late
initialization (when initialization is done before all members that will
hold the settings have been created)
SettingProvider tests.
Use setting providers in ContextHandler.
Only draw one label for combo boxes.
Raise exception, not exception info.
Copy files from orange2 with applied 2to3.
Fixed inconsistent use of tabs and spaces.
Apply consistent code formatting.
Cleanup scaling.py.
Cleanup owparallelcoordinates.py
Cleanup owparallelgraph.py
Cleanup viswidget.py
Cleanup toolbars.py
Orange3 syntax for getting numpy array from Table.
New style widget attributes.
FIXME: Do we have row ids in orange3?
FIXME: what does getitemsref do?
Converted showAllAttributes to new style setting.
Extract None checking checksum.
Enable zoom toolbar.