#266102 by Pancho: (minor) Correct and improve descriptions in arguments' and contexts' settings forms.

#260221 by sun, Pancho. Primarily codestyle cleanup; also fixed some missing leading slashes that were causing the edit icons not to appear in the new helptext for the display editor.

#260358. Added explanatory help text to several core Panels arguments plugins explaining how restriction by node type works.

Major rework of context; fully implemented multiple context, context conversion to arguments and now Views can accept contexts. This is a major step toward turning context into a Fully Operational Battle Station.

Arguments, phase 3 of 3 -- some cleanup is necessary and the taxonomy argument content type still to come