Added gui.labelledSlider
Disable the Entropy-MDL if data does not have a discrete class.
Fix for SQL data.
Remove copies of included data sets.

They should be available from the standard location.
Fix in 'Discretize' widget.

Explicitly change the 'method' index when disabling MDL Entropy button.
Reset the variable states after doing this (and not before).
Added 'Distances' widget.
Fixed Results.get_fold method.
Widgets with context settings now get properly initialized Context
Widget should not set current_context.

SettingsHandler.open_context does that.
SettingsHandler is set by metaclass so it is reasonable to expect it there.
Set current_context to None when context is closed.