Renamed __slide* members to __drag* to clarify their connection with the drag* methods.
Fixed NumericWidget dragBegin errors.

It was returning False rather than None when it wanted to reject drag starts, and instead was starting a drag with BoolData( False ) as a result. This problem is non-existent on the C++ side because of stronger typing, but some "helpful" value converters in the Cortex bindings cause the problem on the Python side. Added some helpful warnings in the Widget documentation to hopefully avoid the same mistakes in the future.
Fixed circular references in plug popup menus.

These were causing the dreaded "RuntimeError: underlying C/C++ object has been deleted" errors by keeping PlugValueWidgets up past their bedtime.
Merge pull request #301 from johnhaddon/dragDropFixes

Fixes for "underlying C/C++ object has been deleted"